要…了【yào … le】

要…了 says something is about to happen. The 了 is part of the pattern and cannot be dropped.

The 了 is what makes it imminent

Without it, 要 is just "will" or "want to". 我要走 is a statement of intent; 我要走了 says you are leaving now.

With adjectives

No specific time

要…了 takes no time phrase. míng tiān明天yāo要xià yǔ下雨le了 is wrong for a forecast a day out – say 明天会下雨 or use 就要…了, which does accept one.

A time word that means "right now" is fine: 马上要开始了.

Questions

The related patterns

就要…了 is the same thing with a time phrase allowed; 快…了 is the same thing without 要 and with no time phrase at all. All three are interchangeable in ordinary speech.