Why connect arduino with Rasberry pi?
Interfacing hardware is easier with the Arduino: more available I/O pins
capable of sourcing and sinking more current, 5&nbsp;V I/O, analog
inputs, very predictable timings, etc. The Raspberry, on the other hand,
is a real computer with a real OS, has a lot more computing power
and memory, has Internet connectivity, can easily run “standard” web
servers like Apache or nginx... or mostly any Linux software for that
matter.

Some projects can benefit from using both. E.g. if you want to make a
web interface for some low-level hardware sensors.