Because SC2 is so economy based, for any plans to be competitive in the middle game it does lock down that early play to be more about what you can do between SCV spawning. So an early rush is cheese if it screws your economical development and so is an all or nothing push within the first few minutes but leaves you dead if it fails (some players think cheese is a sign of a bad player, although if it wins a lot at the sharper end of a tournament this will be taken seriously before a counter to it is built into the typical opening build orders). All the good players scout to know what to expect, most harass to try and psych their opponent into an early mistake (and gain a lead/maintain their knowledge of what the enemy is doing). You start off with a few SCVs and so can scout from second one, once you have a few offensive units then you can make a pain of yourself (Hell, Protos shields mean their probes can annoy an SCV when doing the initial scout and get away scot free. Harass and a scout in one early move).
At the sharper end of the game (and by the sounds of it the tactics are cascading down very quickly as everyone expects a brutal MP and so reads up before joining in, which is a self fulfilling prophecy and so make the MP a brutal environment for a n00bie) the early builds are all about trying to be able to survive just about anything the enemy can throw at you in the opening while going down the path to your mid-game strategy. Add in positional fighting/debate over how much of the map each player wants to pay to control and the fun of watching replays it born.