for street driving, you never need a coilover. Waste money, waste resources.
if its for street driving, and if you somehow need to get a very specific ride height, you'll have to get a coilover, but a cheap one will do.
if you want to do serious tracking, then you need to get a good set of coilovers. There are many non-performance type of coilover brands out there with no proper engineering and research done. They are put out into the market because buyers have associated coilovers=performance, so many people blindly buy and blindly install. At the track, you can definitely feel the difference between a good set that cost over $10k, and a normal one that cost $1k.
In my opinion, the coilover aspect of sitting the spring does not give you any additional advantage if you do not need adjustability. In a theoratical situation where you have 1 x optimised settings, somehow, from your shock + spring set, it should perform just as well as a set of coilovers.
In any case, depending on the model of car you drive, sometimes coilovers are not possible. You buy a coilover set but actually only the front is a coilover, the rear is still separate. How many people know that?