Caffeine is a vasoconstrictor. Citrulline and glycerol are vasodilators. They work against each other to a degree, which is why the strongest tub on the shelf does not always give the best pump.

Splitting the two jobs frequently beats one very strong product. A moderate stimulant pre-workout plus a dedicated stim-free pump formula usually produces a better pump than the hardest hitting single tub.

The other reasons people buy these:

Evening training, where caffeine has a long half life and sleep matters more than the session.

Cycling off stimulants to reset tolerance, without losing your pump for a fortnight.

Stimulant intolerance. Plenty of people train hard and simply cannot take caffeine.

Topping up a pre-workout that is heavy on stimulants and light on pump ingredients, without touching your daily caffeine total.

Two things worth knowing. Pump is really two separate mechanisms: blood flow widens the vessels, cell volume draws water into the muscle cells themselves. Good formulas cover both. And if your product contains glycerol, drink considerably more water than usual, because glycerol pulls water into muscle cells and without enough available it draws from elsewhere, leaving you flat rather than full.