Stupid or useful? The universe decides.
Because some decisions are not worth more thinking. When two options are genuinely close, the cost of deliberating longer is higher than the cost of picking wrong. A coin ends it in one second. The other reason is sharper: the moment it is in the air you usually notice which side you were hoping for, which tells you more than another hour of weighing up.
This uses your browser's random number generator, which is far more even than a real coin. A physical flip is not perfectly fair — it has a small bias toward the side that started facing up, because the coin spends slightly more time on that face while it tumbles. Spinning a coin on a table is worse still; on some coins the heavier design side wins noticeably more often. The version on this page has no such lean.
Five heads in a row does not make tails "due". The coin has no memory, so every flip is a fresh 50/50 regardless of what came before. Runs feel wrong but are entirely normal — in 100 flips you should expect a streak of five or six somewhere. If a sequence looks too tidy, that is usually the sign something is rigged, not the other way round.
Bad use: anything where the outcome actually matters and you have information you have not used yet.