I don't think there is any special use of the light shovel, as opposed to the normal, heavier one. I believe that Ricci gave the best, most logical answer here, so I just upvoted it and won't even attempt to give another.
I only have one small general comment about the shovels to make, because you asked why we have two of them. Well, I can tell you why I keep two different ones of shovels for my every character:
- I generally use the light one, because I have many low level characters so it's just very convenient to save some capacity (so precisely for the reason mentioned by Ricci below) But each of my noob-chars still keep a heavy shovel in their depot too, because...
- For some inexplicable reason (probably the different ID of the items where only one is accepted) the ONLY shovel you can use on Vigintia island each year in January is the normal one.
(Vigintia island is the one which appears annualy near Thais, during Tibia's anniversary festivities, where you can dig for treasures on the little beach, among other activities).
Many (new or returning) players don't know about it and I had to explain it to countless of them so far, because they were running in circles on that little beach, utterly confused as to the reason why they couldn't dig anything up and find any "treasure". And it was all just because they tried to use the light shovel there, istead of the normal one. Secret Service Tools also do not work there, if I remember well (and TibiaWiki says so too).
It's easy to buy that necessary normal shovel on Vigintia from the npc Vigintius, by the way, but there is always that one or two players (at any given hour there) who just do not know what to do.
But even though there is at least this one case (that I know of) when only the normal shovel works, I don't know of any place where only the light one would work or cause any special effect.