VSWR at TX is a bit as efficiency for car engines. If you have a lot of fuel and an engine that is uncritical in its function, you will get needed result at cost of the fuel (with an less good VSWR).
It is mainly for small battery powered transmitters best efficiency are of importance, but also some very powerful transmitters need to keep losses extremely low as they else not can keep temperature within safe range for the electronic circuit.
VSWR at RX is a similar situation which effects signal/noise ratio. Is every dB counted to be able to receive a signal, must VSWR be low.
High VSWR due to poor matching at antenna will result in that for example outside of a coaxial braid will be a part of the antenna. Can increase problem with near-field noise or reduce effective overall gain.
VSWR is a ratio and often expressed as a such.
Expressed as dB is it usually called "return loss" which is a positive value. Negative loss is an amplification.