Katherine is correct. Once you repair the bridge, you will have access to the plots on the other side. Some (maybe all) of these plots require guardian seals to gain access. Some of them require both guardian seals and the "clues" that you are used to using to gain access to the plots on the edge of the estate.

These are also required for upgrades for some of the items (the higher levels of the garage and pagoda for instance), and as Ken mentions you can use them to open some of the "around the world" quests. I would save them up for opening up the plots once you cross the bridge. The guardian seals are probably the hardest item to obtain in the game.

The seals also can be used, if Your time for an "around the world"-quest is over. You`ll need 5 seals in this case to reopen one of the chapters

I have more than the allotted amount in the inventory When are we going to be able to use them?

I don't know for sure, but I believe they will come into play later when they let us cross the broken bridge.

ditto

I'm clueless on this one too!