why these tests ?

All the tests listed on the menu are usefull tools for measuring the degree of randomnes of a given series. So you can choose a test, enter a series of your choice (elaborated by you or with the help of a pseudorandom number generator) and check out if the series passes the test, that is if according to it the sequence has an "acceptable degree of randomnes"; in other words, if the input series resembles a trully random series as it is defined in a mathematical way.

One of the main applications of these tests is to check if a selected PRNG works well, if it has been well constructed and generates "strong" pseudorandom number sequences. In cryptography, it's usual that an encryption algorithm has a PRNG inside its code. And as you know, it's impossible to create a good cipher if it is based on a bad PRNG. We cannot state the same vice-versa, as a good PRNG doesn't guarantee that using it in an encryption program will yield a good ciphertext. But since the absence of a strong PRNG leads nowhere, here we have some tools to check out if a PRNG is acceptable to some degree of certainty; or more precisely, tools that enable us to state if a PRNG is bad, but without mentioning if it's good in any case. Despite this "pesimistic" approach, wich is mandatory when focusing on this topic, what is true is the fact that a single test passed by your PRNG doesn't mean too much, but each new test that is passed, makes it a bit more probable that the PRNG is satisfactory. So, use the tests as a statistical tool to measure the quality of any PRNG or sequence of your choice.