Comment tags are not visible to the reader because they are hidden in the HTML code. They provide a way for the web master to make notes right on the page. Although the reader doesn't see these tags, some search engines, such as Inktomi, can index them.
Therefore, comment tags are another great way to add keywords to your page, thus increasing keyword frequency, an important factor in many ranking algorithms.
Avoid the use of spam
Search engines do not penalize for the use of comment tags. However, the general rule is not to repeat the same keyword more than 3-7 times. So, to be on the safe side, follow that rule in comment tags.
Place them on every single page
Add keywords to comment tags
Place them after meta tags in the head
Do not repeat keywords more than 7 times