Something I've noticed at several live shows (especially blues and jazz) is these short plexiglass walls placed in front of the guitar amplifiers. Why would they do that? What are they for?
They are used to manage the on-stage volume levels while still allowing the amplifiers to be cranked up for tone. These shields limit the amount of sound directly hitting the on-stage performers. You will also see them around drummers.
This type of shielding will also reduce cross-talk, e.g. preventing the drum sounds getting captured by the guitar microphones, but I haven't heard of this being the primary driver for on live shows; but they have been used in studio situations to reduce bleed.