When I used to play, many, many years ago, all that running around for 90 minutes made my mouth feel "claggy", if that's the right description, and spitting helped clear it for the next lot
. I suppose your mouth dries out as your are sweating and losing fluid as you aren't taking liquids on board. So, I understand why.
However, last night I saw Slaven spit and I though he's not running around for 90+ minutes, but then I realised, he kicks ever ball, heads every ball, runs and covers every inch of the pitch during the game from the technical box and I thought, you know what, he's entitled to spit.