What are shareholders supposed to care about? They give a company capital in hopes of getting more back in the future. Are shareholders meant to be philanthropists?
Aka capitalists
You buying 5 shares shifts the demand curve for the share to the right, resulting in an increase in the price. (absent a simultaneous proportional increase in the supply of shares).
While it may not feel like your purchase of 5 shares gave Apple capital, your demand was surely part of the aggregate demand and supply that determines Apple’s share price at a certain point in time, and hence informs Apple’s market cap.
Point 1: increased demand for stocks will increase the price of it and thus the market cap. Point 2: the higher the market cap of a company the more capital it has available.
Yes, it theoretically means they could raise more money by issuing shares, but that's not happening.
But they don't have to do that. They could issue the shares for RSUs. If they issue new shares (either to sell to the market or to use for RSUs) then the market cap directly influences how much they can generate in work or cash from such a transaction.
Also, price is not based on market cap... market cap is based on share price and count. But the larger the market cap, the more money can be raised by selling shares without diluting existing shareholders too much.