多一點 with no 了 asks the quality up a notch (give me a bit more). 多了一點 with 了 reports the quality has already gone up a bit. The 了 turns a request into a change that has happened.
有一點 sits before the quality and reports a mild, unwanted amount that is simply there (有一點貴, a bit pricey). 了一點 sits after it and reports the quality has risen a little from before (貴了一點, has gotten a bit pricier).
A比B
比 raises one thing against a named other (A 比 B 多). Vs了一點 raises the quality against its own earlier level, with nothing else named.
一點多了 ✗ → 多了一點 ✓
asking instead of reporting: 請多了一點 ✗ → 請多一點 ✓ (drop 了 for a request)
多一點了 ✗ → 多了一點 ✓ (了 hangs off the verb, before 一點)
English folds the change and the amount into one comparative word (a bit longer). Mandarin splits them: 了 carries the change, 一點 carries the small size, in that order behind the quality.