Sidechains could unleash even more innovation from bitcoins blockchain, in turn making bitcoin more valuable too.
A side chain is basically an altcoin, but with a key difference: the ledger units can be pegged to some amount of Bitcoin. This means that the altcoin would essentially be using BTC instead of a different currency. That eliminates the currency exchange risk that one must incur when switching between altcoins, opening up all cryptocurrency innovations to bitcoin users.