Sample Rate (Hardware)

Only a block that represents some underlying hardware with its own clock (e.g. USRP, sound card), or the Throttle Block, will use 'Sample Rate' to set that hardware clock, and therefore have the effect of applying rate control to the samples in the flowgraph.
A Throttle Block will simply apply host-based timing (against the 'wall clock') to control the rate of the samples it produces (i.e. samples that it makes available on its outputs to downstream blocks).

Only a block that represents some underlying hardware with its own clock (e.g. USRP, sound card), or the Throttle Block, will use 'Sample Rate' to set that hardware clock, and therefore have the effect of applying rate control to the samples in the flowgraph.