I wonder when it's preferred or better to use subequations instead of normal equations. I.e. when to use 'normally' numbered equations (e.g. (1), (2), ...) and when to use subequations (e.g. (1a), (1b), (2a), (2b), (2c), ...).
I can think of a few situations when to use subequations:
Expansion/abbreviation, e.g.
We have $$y = x+x,\quad(1a)$$ which can be simplified to $$y = 2x.\quad(1b)$$
Combination of equations, e.g.:
We calculate force F using $$ F = m*a,\quad(2a)$$ and work W with $$ W = F\cdot s.\quad(2b)$$ Combining eqs. (2a) and (2b) gives $$ W = m\cdot a\cdot s.\quad(2c)$$
There are other examples, certainly. I'm not sure why 'normally' numbered equations without letters aren't simply used. Is there a general guideline or best practice?