What is Null value ?

Nulls are neither good nor bad. NULL is simply the absence of data. How one deals with NULL is entirely dependent on the environment you are working within. The various forms of VB and xBase platforms seem to be  the worst at accommodating NULL within code. So, from that perspective, . However, from a database perspective (excluding MS-Access) a column having a NULL state is actually useful information (the absence of a value).
I find NULL to be a useful data state, in that this clearly indicates that a value has either not been set, or specifically cleared.

It's a pain to deal with fields that can either contain a value, blank, or null. It's just a bunch of extra coding to deal with the variations when writing code.
It is different from dealing with results from a query where there is a left join that returns a null value.