The only reason to buy would be if you lived on a flat or something and it was really awkward to setup an R.O
Other than that why would you even consider buying it.
If you can get it for a good enough price it does cut the hassle of having to keep an eye on your system etc.
I had a 1000 litre tank in the garden for years, with all the associated hoses. Filling up at the unit is a doddle for me.
When we sell water our aim is to make it only slightly more expensive than making your own.
In WFP there are loads of ways to save money that involve a little bit of thought and a little bit of time, but we don't all do all of them.
Making your own suits some people and could save a tenner here or there. Depends if you want to spend time doing it, not everyone does.