A good immigration consultant is very useful when:
1. You are unable or unwilling to go through the sometimes complicated immigration process without professional help. Maybe you work 60 hours a week and you don't have the time to read through everything. Maybe English or French isn't your first language and you find it hard to understand complicated documents. You may be fluent at speaking it, but parsing a legal process.. maybe not. A small mistake can have big consequences so if you're not comfortable scouring forums and asking tons of questions, a consultant/lawyer makes sense.
2. You have a complicated profile. Maybe you have a criminal record and you don't' know how to apply for rehabilitation. Maybe your spouse was denied multiple visas. Maybe you're divorced/separated with a kid and you need to prove that you're the legal guardian. There are all sorts of tricky cases where you should absolutely consult a professional, either a lawyer or a consultant.
3. Peace of mind - perhaps you understand everything and you have prepared your own file, but you just want a pro with lots of experience to look at it and give you qualified, expert (and paid for) feedback. You may want to know that everything is perfectly in order before you submit. Again, there are obscure regulations and situations which an expert may catch while you miss it. Basically the same reason you can change the oil in your car, but you don't want to touch the brakes unless you really know what you're doing.
And any combination of 1, 2, and 3.