The first date is absolutely pivotal in setting the stage for the rest of the relationship. For starters, it defines whether there will be a relationship in the first place. Since the prospective partners usually know little or nothing about each other, the first date is about getting to know you, seeing if there is chemistry or not, etc. A first date starts to set the tone for the rest of the relationship out of what is discovered by each other in the 'getting to know you phase'. Through what would be presumed as initial kind or polite interactions, it will really be found out how long and how strong the relationship really will be.
In that first date, initial parameters of the relationship start to form. An early, more embyonic concept of what the relationship will really be like starts to form. Words, phrases, body language starts to hammer out the concept of a possible relationship. It may also fail, making the first date ultimately the last one. But ultimately whatever comes out of that first date sets the stage for the future: be it a short-term, mid-term, long term, or no relationship whatsoever. Decisions are made by the first interactions on where and for how long the relationships are going to be.
When people meet for the first time, even if there is a spark, or other form of instant chemistry, the first words and actions easily sets the stage...For example, if a male makes a sudden amorous move on the other partner, it could be viewed as a very negative thing and shorten the date to one or none. Excuses could be made for the prospective partner away from the amorous advances of the other, and then phone calls or emails are not returned. Sometimes, the date is just for sex (ie a one-night stand,) but even if that is the case, it's also good to explore the parameters of that, as we live in an era of date rape, stalking, and obsession.
First dates set the parameters for everything that follows. We all need them, even if there is strong chemistry, to find out what the future may be with that person. It gives us a period of time to see what the person is like, are they suitable for us, etc. In that brief period of time, both prospective mates define what the future may be with each other. If after talking, and there can be no future, parting of the ways usually is the way, sometimes nicely, sometimes rudely; then, the search begins again.