So that reception center you booked already has an on-site coordinator, and you're wondering what you would need a wedding consultant for? Let me clarify that and point out some of the differences. Let me preface though by stating that I do not have any problem with on-site coordinators, and actually love working with them! They make my job easier because they are my "go to" person, rather than having to deal with many people.
1. An on-site coordinator is obviously employed by the venue and is there to coordinate the details of your day as they pertain to them. The venue pays them to make sure things run smoothly for them. A wedding consultant is paid by you, making you and your happiness their main concern. They are your alliance and voice and will make sure things go as you planned.
2. On-site coordinators give you vendor referalls based on their preferences, vendors who work well with that particular venue. Wedding consultants have lists of vendors that fit every couple's budget and personality.
3. On-site coordinators are there primarily for the day of only, they are not there for your unlimited emails and phone calls like a wedding consultant. On-site coordinators will not attend vendor meetings with you or confirm with your vendors like a wedding consulant. A wedding consultant is there for you every step of the process.
4. I am not aware of any on-site coordinators that will decorate for you. They will not string up those chinese lanterns, tie your chair sashes on, put out your centerpieces that you made, etc. Most wedding consultants will do that, part of the job is to oversee the decor and design of your reception.
The bottom line is that your on-site coordinator is there to gather the main details from you after you have planned and execute them, such as start time, catering menu, number of chairs needed for ceremony, and those kinds of things. A wedding consultant wants all of your details, to make sure things are just as you planned and envisioned!
I hope this helps clarify the difference and helps you to see that the two are not the same. Happy planning!