In a simple way, there is one thing that all traders care about, and that is where they entered and the profit or loss of their current position.
So price itself is the thing that everyone can see and care about, and highs and lows are something that everyone can see and care about. That is why they are so important as has been mentioned above.
When you talk about an indicator like RSI, or MACD or whatever with settings, does the 'market' actually care? If for example the market generally cared about the 200 day moving average, that would make it useful, but if you pick an indicator with arbitrary setting like RSI(14) and MACD(12,26), who is actually looking at these with the same settings as you?
Also as has been mentioned above, an indicator could be useful as confirmation of bias, a trend. They can also be useful to measure volatility, because the market can be very wild at times and calm at others.
But do not use some indicator with arbitrary settings and expect that it is a good way of understanding or trading. Traders bought at 10, the price is 9, they care about 10 and 9 (the current price), they do not care what a stochastic oscillator says about it.
Probably the best route is to remove all indicators, and in particular all indicators with arbitrary number settings.