It depends on what level you are and what are your goals in the game. I am at XP 60, VP 930-960. The statue storage is extremely useful for quickly rebalancing your statues. For example, when Mega Crab arrived about a day and a half ago I ditched 50% RR MP and put an extra 14% TH guardian, which was in the statue storage. The Crab provides so much resources that RR MP is not needed. However, any extra attack related statue is of great help. I chose TH guardian but I could have also put 15% GBE guardian, which I also had in the statue storage. Once the crab leaves I can imediately put another (41%) RR MP, which is also in my statue storage. Once I no longer need RR MP (in about 3-4 weeks from now and once I max out my last troop, tanks), I can immediately put PSC MP (I have two in my statue storage) or roll back to whatever I decide at that point. I hope that helps. 

However, even a lower levels it is extremely useful. Let's say that you get a great statue (whatever it is) but you cannot place it yet becasue you cannot (yet) upgrade your scupltor. You can store that statue and place it once you upgraded your sculptor!

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