First off, a distance of two semitones is a major 2nd while the distance of three semitones is a minor third or could be an augmented second.
How we name intervals is very useful for constructing chords and scales since we have 7 letter names(A to G) for notes out of the 12 we use and typically only have the distance of a whole tone (or two semitones) between notes in a scale.
Let's look at this solely from a distance perspective at first. The distance from unison to octave are as follows in semitones:
0 - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12
In C these notes would map to:
C - C#/Db - D - D#/Eb - E - F - F#/Gb - G - G#/Ab - A - A#/Bb - B - C
As you can see, both 0 and 12 map to C and the furthest you could be away from a C in semitones is 6. This leaves us with 5 notes on each side with 1 - 2 - 3 - 4 - 5 closer to 0 and 7 - 8 - 9 - 10 - 11 closer to 12.
Now let's look at the standard interval names. In this I will use M for major, m for minor, P for Perfect and tt for tritone (which is considered both an augmented 4th and a diminished 5th).
P1 - m2 - M2 - m3 - M3 - P4 - tt - P5 - m6 - M6 - m7 - M7 - P8
There are a few things to notice that should help you understand intervals better.
- The tritone (or 6 semitones away) has a perfect interval above and below (P4 - tt - P5).
- The note you are basing the name off of (C in this case which is both 0 and 12) is also perfect (P1 for unison P8 for octave) .
- The other notes are group into twos (because of the two semitones typical max in scales) with the smaller one being minor and the bigger one being major (m2 - M2 - m3 - M3) and (m6 - M6 - m7 - M7).
When you look at it this way, you can see the symmetrical nature of intervals and how they are used to build scales.