So if you first play with interface for few days, but end up not attempting to write more of your great next novel for two months because you were swamped with real life, you can come back and there's still most of the trial left.
Just because you offer a 7 day trial and I had time today to try it out a bit, doesn't mean I'll necessarily have any time at all over the next 6 days to finish evaluating it. Life and other work priorities happen.
After 30 days of ACTUALLY using it (days which are sometimes few and far between, and sometimes more closely spaced) is really a point at which, yes, it's justified to purchase, showing that it has been "the tool of choice" so many times, and likely to be so into the foreseeable near to mid future, too.
The trial is critical to a) proving that it does the useful things, b) determining that that it does said specific things better than the alternatives for some relevant definition of better, and c) giving enough of a chance to really learn it well enough to make an informed decision.
The free period builds tremendous goodwill, and is a really sane and "nice to the community" choice for a tool that might get used occasionally. It shows the confidence that the market really is there for it in general. People who can/do reap value from it on an ongoing basis will convert.
The current SaaS model is like going to the store and you can only buy 5 gallons of butter or milk and you have one week to use it. It "feels" like most of your money is being "wasted". At least that's my perception.