FP allows you to return at a set time... you get to avoid standing in the line, but your return time is almost always going to be later than actually waiting in standby.
DAS will allow you to return at a set time.. (similar to FP).. you get to avoid standing in the line (similar to FP), and your return time will be set based on the standby time, not the FP return time (so generally better than FP)
FP is available to everyone.. but is limited in it's availability... it runs out. DAS won't run out..(better than FP) you'd be able to use it on-demand. The limit is you just can't stack DAS waits... but you are not limited to what you do during your wait. So you can go on other rides (like FP), or even get another FP while waiting (better than FP).
Think about how everyone complains 'if I don't get there early.. I won't get a FP'. Imagine never having to worry about that anymore.. and basically being able to use a FP-like service anytime you want for any attraction.
Now think about FP+ being limited to 3-4 uses in a single park per day. DAS will not be limited in how many times you can use it. You will be able to park hop with DAS and use it in both parks.. unlike FP+
Starting to see the attraction for people to lie to get a DAS still?
It's like being able to get a FP for any attraction