Why bloodwork catches what symptoms don't
Kidney and liver function, thyroid levels, and medication side effects often don't produce visible symptoms until a condition is fairly advanced. A blood test is frequently the only way to catch a real problem while it's still easy to manage.
Why it's specifically important on medication
Several long-term medications require monitoring for exactly this reason, to catch a side effect early, or to confirm a dose is still appropriate as a pet's condition or weight changes. Skipping the test doesn't mean nothing's happening, it means not finding out until later.
What actually happens when it gets skipped
Usually nothing, for a while. Then, occasionally, a problem that would have been caught early and managed simply gets caught later instead, at a point where it's harder and more expensive to treat. The test itself is the cheap part.
How to actually not lose track of it
The moment a vet mentions a follow-up date, it belongs somewhere more reliable than memory. Halo's calendar keeps recurring appointments visible well in advance, the same system behind keeping vet contacts and key dates ready generally.