Cronbach's alpha is used to assess an instrument's

Cronbach's alpha assesses an instrument's reliability, specifically its internal consistency.

It gauges how closely a set of items measure the same underlying construct.

It does not directly assess validity, which is whether the tool measures the intended thing.

It does not measure randomness of scores.

It has nothing to do with the longevity of the instrument.

Alpha values above roughly 0.7 are usually treated as acceptable.