Consider a scenario where you have a cash account with say 100 dollars and you buy 1 share of stock X for 100 dollars. That's it, you can no longer trade for two days. Because trades take two days to complete, if you have a cash account you need to wait that full two days before you technically own the shares and can sell it. Until that two day elapses, your account has 0 dollars in it.
This is known as free riding:
https://en.wikipedia.org/wiki/Free_riding_(stock_market)
Now if you have 100 dollars in your account and you buy stock A for 30 dollars, then your account will have 70 dollars left that you can use to trade with, but this is an incredibly inefficient and impractical way of trading.