What it actually does to older pipes
Most chemical drain cleaners work by generating heat through a chemical reaction to break down the clog. That same heat can soften or warp older PVC and, on galvanized or older metal piping, the caustic chemicals accelerate corrosion. If the clog doesn't fully clear on the first try (common — it often just clears a channel through the clog rather than removing it), you've now got a pipe full of caustic chemical sitting there, and a plumber snaking it afterward is doing it with that chemical still present.
It usually doesn't fix the actual problem
Chemical cleaners dissolve organic buildup like hair and grease reasonably well, but they do nothing for a lot of common clog causes — a toy or object lodged in the line, a bellied or offset pipe section, or root intrusion in an older sewer line. You can pour an entire bottle down a drain with one of those causes and get zero improvement, just a pipe full of chemical and a clog that's still there.
What we actually use
Mechanical snaking clears most standard clogs without touching the pipe chemically. For tougher or recurring buildup — grease-heavy kitchen lines, older main sewer lines with buildup along the walls of the pipe — hydro jetting uses high-pressure water to scour the full interior of the pipe clean, not just punch a hole through the middle of the clog. It's more thorough and it's not corroding anything while it works.
If you've already used a chemical cleaner
Tell whoever comes out to look at it. It changes how we approach clearing the line safely, and it's not something to be embarrassed about mentioning — almost everyone's tried it before calling a plumber. We'd just rather know than find out the hard way.