Quick answer: Select Linked with L can select all connected vertices/edges/faces by just hovering your mouse over it and pressing the key, whereas Select Linked All with Ctrl+L needs a selection in one way or another first and then selects all connected vertices/edges/faces of this selection.
The longer answer, additionally to what I've said before: Since Ctrl+L needs an initial selection first, to add to an existing selection you have to make a new selection while holding Shift (if you don't want to lose the previous selection) and then press Ctrl+L again. With the L key you just need to hover over a different new part to add it to the existing selection, the previous one will be kept.
The very important difference now is, since Ctrl+L selects all connected vertices of an existing selection, it selects also those parts of a mesh which you may have left intentionally unselected.
Example: You have the following mesh as shown in the screenshot. On the sphere there is an already selected area. Now you also want to select the two separate parts on the right.
If you would like to do that with Ctrl+L, you would have to hold Shift and select at least one vertex on each of those pieces. But now comes the problem: Select Linked All selects all vertices connected to the selected ones. The result would be the following:
Let's say the mesh would be more complex and you had a hard time making the selection on the sphere and intended to keep the unselected areas, Ctrl+L is not the right tool because it selects every connected part.
If you instead move your mouse first over one piece on the right, press L, then the next and press L again, you get this:
Another thing is, maybe you accidentally hit L over the wrong piece - which can easily happen in a complex or confusing mesh without manually selecting a vertex beforehand - you can simply deselect it by hovering over it and pressing Shift+L. This is not possible with Ctrl+L, there you have to undo your action which might be very helpful if you realize it late that you have a wrong part selected.
The advantage of Ctrl+L on the other hand is, you can have very many single vertices selected through whatever option, tool etc. and with one step you can select everything connected which might take more steps with just using L. The best is working with both tools in combination, I guess.