Why do soccer players spit all the time when playing?
I used to spit in my gloves. Most modern goalkeepers (I.e youth players today) never knew the struggle of keeping sticky gloves because gloves today are like glue on the palms.
But in dry conditions, spitting on your gloves gave you a lot of added grip. That was passed down to us by the older generation (mind you, I'm only 25, so I'm talking about older coaches when I was 16 or so). The older generation played in goal with out gloves, so spitting in your hand and rubbing it around gave you more grip to catch a ball. That transferred over to gloves when they began being used.
My teammates on the pitch would spit because you get "sticky spit" which a runner doesn't like (but can be a godsend if you're a keeper and you have a big wad of sticky spit in your mouth..oh the glorious grip that came with that). As others have posted, it makes your mouth feel even MORE dry, and it gets more difficult to breath properly when you got mouth boogers blowing down your throats when sprinting.