Why must use 5A fuse when maximum current is 4.33A?

Generally speaking, you need to account for any variance in electrical output. There are a lot of factors that can cause a fluctuation in electrical amperage. You wouldn't want to set the fuse too low or it would blow needlessly, but you also don't want to set the fuse too high, or a dangerous short will not be interrupted.

A power source/device can produce/draw more amperage while damaged than in normal operation.

A 4.33 amp device might not run at 5 amp while working, but while damaged could exceed that dramatically.

When looking at the reason for having a fuse, it provides two things:

1) help protect a working device from having too much amperage forced into it from a damaged power source

2) help protect the power source from having too much amperage draw from it by a damaged device.