Because it translates human behavior, and human behavior is what moves the markets, it doesn't matter if you are an institutional trader or a self-employed trader: you want to buy lower and sell higher. Hell, even automated trading is (sometimes) programmed to think and see price action like humans, with the exception of most high frequency trading. Now, large institutional market movers can't just hit "fill 10000000000 shares", they do it slowly in a timed fashion, in part giving rise to the classic trend geometry we all are used to see and follow. There are interesting techniques institutional movers use to fill whatever they need to fill without causing strong market impact (Google it) because, after all, they also want the best prices they can get.
Technical analysis is nothing but a reflection of human behavior, human expectations and fear. Now, there are deeper issues and limitations of pure and naked technical analysis. Sometimes opposing market movers try to manipulate prices up or down, but we can see such activity with advanced tools. They call it tape reading but it's very different from the classical tape reading. Tools like volume profile are very useful.