Can all primes be written as a Mersenne prime? Do all primes can be written in form of Mersenne prime? 
If not, why Mersenne form is important?
 A: If all primes were Mersenne primes then there would be no need for the term "Mersenne prime" to even exist. Mersenne primes are special primes that are of the form $2^n - 1$ for some positive integer $n$.
Actually it turns out that the only way such numbers can be prime is if $n$ itself is prime (but the converse is not true, e.g. $2^{11} - 1$ is not prime).
The prime $5$ cannot be a Mersenne prime since $6 = 5+1$ is not a power of $2$.
As for your second question, the answer above tells you the applications to primality testing, it is easier to study the primality of such numbers in general.
Another application is to do with perfect numbers. A number is perfect if the sum of its proper divisors is the number itself. So for example the numbers $6$ and $28$ are perfect.
Now it is possible to show that an even positive integer $n$ is perfect if and only if $n$ is of the form $2^{p-1}(2^p - 1)$ for a prime $p$ such that $2^p-1$ is prime. So finding Mersenne primes is exactly the same as finding even perfect numbers.
A: Trivially, the answer to the first question is no, since for example $13$ is a prime which cannot be written in the form $2^n-1$.
For the second question, one of the reasons that they are considered useful is that there is a relatively straightforward way of testing if a Mersenne number is actually a prime or not. This process, the Lucas-Lehmer test is not difficult to turn into a computer algorithm, and it can be used to search for Mersenne primes of enormous size, so that almost all the numbers which have ever held the title of "Largest Known Prime" have been Mersenne numbers.
