This is widely believed but scientifically unproven. The understanding of the significance of awareness of heart rate to workout performance is ongoing. Basically, tracking heart rate shows what is already known, that improving cardiac performance will improve resting heart rate. The stated purpose of doing so is to increase self-esteem. For the same reasons there are large mirrors installed at most gyms.

> Also, accurately measuring your times, and seeing even few seconds improvement since the last week's training helps keep you motivated.

A $5 stopwatch is accurate to hundredths of seconds.

> There are many reasons to use such accessories, other than "stroking vanity".

This is an appeal to common sense, aka the fallacy of axiomatic thinking, or unsupported assertion. Claims which can be asserted without evidence may also be dismissed without evidence.