Gaiters are designed to keep the lower leg and socks dry.
They are ideal wear when walking through wet undergrowth or shallow snow.
Although we took gaiters on our 2007 TMB, we found that we either faced torrential rain or cold wet fog and so in both cases preferred to wear our waterproof trousers.
That said, we noted some people hiking in gaiters.
There appears to be a prevalent view that waterproof trousers should only be worn whilst it's raining. Otherwise wear gaiters.
So it really is down to personal preference and judgement.