Watch this animation, which shows an angle opening out.

It stops at multiples of 90°.

Look at this diagram, which shows four right angles placed next to each other at a point.

Here is 1 right angle, which is 90°.

Here are 2 right angles put together, giving an angle of 180°.

2 × 90° = 180°

Here are 3 right angles put together, giving an angle of 270°.

3 × 90° = 270°

Here are 4 right angles put together, giving an angle of 360°.

4 × 90° = 360°