If you've read this far without becoming intensely frustrated, you'll realise that I feel that tools have no inherent power: instead, they're empowered by those who work with them. Since ritual itself is only a tool, then, why does anyone need to use it? If you've read around the rest of the site, you've also probably found that several people who share it with me would simply say, "you don't need it". So, to perform the standard scientific trick of rephrasing the question so that one can give the answer one wants, I'll ask instead "Why might someone want to use ritual."
First let's define what I mean to use it for. What we do has variously been described (by practitioners like Crowley and Farrar) as "the art of altering reality (or sometimes consciousness) in accordance with will" and "the art of achieving communion with one's guardian angel (or sometimes 'inner self')". It's hard to challenge either definition - and there are others probably equally valid. The mechanism for achieving this aim is usually stated quite simply along the lines of: "carefully formulate and state the intention; concentrate the will and/or raise the power; apply the will/power to the intention".
Easily stated, not so easy to actually do. We may need a little bit of help at times and ritual can, on occasion, make life easier. Ritual can ease the process of something as simple as getting up in the morning. State the intention - "I want to get into work on time". Summon up will-power - say after me: "I do have the energy to get up". Apply the willpower... But, it's dark and cold and another few minutes won't make any difference - I can catch the next bus. Now try the morning ritual. Lie in bed until the weather forecast on the radio. Imagine that steaming mug of green tea, the first one, the best cup of the day. Get up, set the kettle to boil, spoon the tea into your favourite mug, kept for the purpose. You get the idea.
Ritual for magic is also like that. The act of preparing a sigil can help you in stating the intention very precisely. It may mean that you think about the intention in a new way. That a biopsy doesn't find cancer is a laudable aim to work for, but just perhaps the manufacture of a bindrune will remind you that it would be better to work for the biopsy confirming that the patient doesn't have cancer.
The act of anointing a candle can help to get you into a headspace where it's easy to raise the will to empower that bindrune. In fact, if you've done the ritual a few times before, you may find that you just slip into the headspace without noticing, in the same way that you were (hopefully) already out of bed before boiling the kettle.
Perhaps most importantly, the denouement of the ritual help focus the willpower onto the intention. A ritual drama reaches a climax - even a catharsis - for a reason. Every bit of the energy that you've built during the familiar actions and words that you've performed are focused onto the bindrune at a pre-determined moment, so that there's no doubt about the "when" of it. With the doubt removed, "now" you visualise the fire emanating from the grooves, and "now" you visualise the energy doing - well, whatever you want the energy to do. It's just a tool, but it's quite an effective one.
With groups, ritual is perhaps even more useful. Some people on this site work exclusively alone. I can do that, but I also find working in a group effective. Hell, I actually enjoy it. But if it's difficult to state, concentrate and empower as an individual, it's even harder with a group. Ritual helps to synchronise the intent. The high priestess ideally uses a form of words designed to be precise enough to ensure everyone means the same thing, whilst just ambiguous enough to allow them to visualise it in a way that suits them.
Ritual helps to synchronise the concentration. The circle is built up gradually, so that everyone gets used to the pace. Or the gods are summoned in such a way that everyone has that sense of anticipation of what will happen next. With skilled design, the group's minds are attuned without them really having to think about it.
And again, perhaps most importantly, ritual synchronises the empowerment. Maybe the group holds hands, spinning faster and faster until they ground the power by bowing their foreheads to the earth. Whatever, there's no doubt about the "when". The effect is one of "I don't really know where the power came from, but I sure know where it went."
Particularly in a group, you should be prepared to (and that means also having someone with the skill to) change a ritual to suit, or even modify it on the fly if it's not working. That's why there are often defined roles, with, say, a helper to ensure that the priestess can concentrate on what she's doing, while the "handmaiden" ensures that the timing and flow are right. When it works, it can be effective, powerful and beautiful.
But don't become entranced by ritual for its own sake. The adept can use anything, but is reliant on nothing. You should be able to banish something without a crate full of tools and a chalk circle. You should be able to defend yourself against a marauding whatever-it-is using just your mind. And that means that, once ritual works for you, you should be able to lie in bed and imagine the candle. And, as a group, you should all be able to imagine the same candle, which may be a little harder.