I have noticed that often at the very start of the match, alot of people will send a worker to run around your base. Usually this is way before the stage where you build any structure (e.g. roach warren) that could actually tell them anything about your strat. My only theory is that they are trying to see if you are rushable or they are trying to get your workers to chase it so you waste income. So why do people do this? Personally I prefer overlords/changelings/obs/orbital command scan/lings/rines/zealots/etc for scouting, as 1# They are better at it 2# They are at a time where they can actually tell you some useful info about your enemy 3# They arent being pulled of your mineral line 4# They are useful for other stuff as well.

I especially love changelings. Nothing says scout like a unit that isnt auto targeted and disguises as an enemy. Also fun to make them dance in the enemy base.

As some players have mentioned, early scouting is just as important as scouting later in the game. From what I can draw in my current experience, early scouting gives me critical information. For one, it lets me know if my opponent is going early gas, which is indicative of higher tier units. It also lets you see if your opponent is deciding to rush you, or perhaps detecting trickery amiss, such as proxy Gateway or Barracks.

Early scouting can, at times, also pressure your opponent into going a different build as you've been able to peek into their current build strategy allowing you to formulate a deterrence for it.