I like lenses in about a factor 1.33 to 1.5 multiplicative progression. Such as 16mm, 21mm, 28, 45, 55, 75, 105, 135, 180, 300, ... etc.
Additive focal length differences do not make sense: a 180 mm and a 200mm lens have nearly the same in field of view, yet they are 20mm focal length apart. What is the difference in field of view between a 15mm and a 35mm? Huge! Both lens pairs are additively different by the same 20mm. The 180/200 pair is multiplicatively apart by a factor of only around 1.1, the 15/35mm pair by a factor greater than 2.3. There is the hitch!
Now a 21 mm is around 1.5 times your 15mm focal length. So, yes: quite a different perspective and quite necessary in my view.