"Hold them in place" is a bit misleading. The molecules in most nanocages generally still react with the nanocage through specifically designed binding sites that actually hold it in place. This means a nanocage is hard to design for nontrivial molecules like folded proteins and it's usually not a perfect cage that completely envelops the target. A lot of stuff is still bound to react with the nanocage and the material inside until it can reach its destination and the cage unfolds.