Sometimes you get a flat - usually it is only at the bottom of the tyre.

Often if you leave the nail in you can limp to the tire fitters and let them sort it. As soon as you remove the nail the tire goes down and if you leave it you risk damaging the sidewall of the tyre.
If a puncture is in the middle third of the tread it can be repaired, anywhere else and you have to get a new tire!

Imagine my horror when I got a puncture in a NEW EAGLE F1 (£90 a go) before I Even got it home!
[bad puns]Getting a puncture is positively depressing. Inflation means it costs more and more to fix. Its not weely funny though.[/bad puns]