real time kernel: benefits

Which is a perfectly valid statement.

But even if you have certain realtime tasks in the system, that still doesn't mean you need PREEMPT_RT.
Rule of thumb: If you don't know whether you need RT, then you probably don't need it.

PREEMPT_RT is *not* a solution that magically makes your machine realtime capable.
Realtime requires that you have a realtime concept for the whole system (including hardware).
The differentiation between "soft" and "hard" is basically useless. In practice what is really needed are numbers for latencies, deadlines and throughput that you have to achieve and then design your whole system accordingly.