It depends on the size of the venue and the type of guest artist you book.

In our area, Edinburgh , Leith, Balerno, and Nitten(usually but not always on guest nights) have some kind of PA system as does Kirkcaldy. Slightly further afield, so does The Star in Glasgow. I'm not sure about Stirling or Falkirk.

There are other smaller clubs e.g. Dunfermline, The Wee Folk Club in Edinburgh, Nitten(On singers night) and so on which do not but these are smaller rooms and, usually, solo guest artists or those who would be able to manage without PA.

In a larger room, it is necessary (I'd suggest) although there are some occasions we could manage without it. However, many touring acts expect this and will often insist on a particular "spec".
Obviously, it also useful if you have a band and require to "balance" instruments e.g. bagpipes, fiddle, guitar and so on.

We're not talking about a giant rig, of course. Just something to balance and slightly enhance the natural acoustic performance.