Not all intervals in the same numerical classification are the same size. This is why we need to specify the quality of an interval by determining the exact number of whole and half steps it contains.

Between any two piano adjacent keys, including black keys, there is a half step.

Some seconds have two half steps or one whole step, while others have one half step. In the following example, the second from C to D is a whole step, while the second from B to C is a half step.

The same thing happens with other intervals. That is why we need to specify an interval's quality by determining the exact number of whole and half steps it contains.