At Cooking Lvl 1-4 you'll get cooking xp from cooking sardines.
Cooking 5-9, you won't get cooking xp from cooking sardines anymore. Now it'll be herrings.
and so on...
The higher your Cooking Lvl is, the easier it'll be to cook sardines. Less fails, less likely to get burnt or less than 100% cooked fish.
Also, higher Cooking levels will increase your fish stacking limit. Higher Suffusencing levels adds to your fish stack limit, too.
Non-suffusenced fish has lower stacking limit than suffusenced fish. For example, I currently can stack up to 545 plain sardines (cooked or not), but if they all are suffusenced, I can stack up to 700 or more. I dunno what the limit is, but it definitely is a lot more.