The nozzle vanes serve to turn and accelerate the flow into the following rotor stage. To do this they need (or produce) some pressure drop in the gas path.
But at the inboard end of the nozzles is a gap between rotating and static machinery. The gap amounts to a leakage or bypass path for the hot gas, and tends to diminish the effect of the vanes.
Thus the lab seals provide as much resistance to the gas leakage as possible. They cannot stop it all, but they're very important to keeping the cycle efficiency up.
Similar lab seals may also be part of the scheme for preventing oil leaks from a sump, by keeping air pressure on the dry side of the oil seals.
