Compression shirts

Hi All

With regards to running tops I have a technical top but does anybody use the compresson tops and, if, so, what are your thoughts on

a) do they give you any benefit ?

b) should you keep them on after training ?

c) I guess they should fit like a second skin (snug) ?

d) any personal recommendations ?

Also with a wicking top, should these be a snug fit as well to keep close to your body to get the sweat away

Thanks in advance and apologies if these seem a bit numpty