Hello Rensi Sam:
If you set the sampling rate of your hardware source/sink block to samp_rate, then it will generate or consume samples at that sampling rate. Ex: if the sampling rate of your Osmocom source block is set to 1e6, that block will generate 1e6 points per every second. In this case, the capability of the radio device restricts the choice of the sampling rates (Ex: USRP N2x0s can only take sampling rate of 100e6 / N (integer), where 4<= N <= 512).
Same thing will happen when you use a throttle block - a throttle block with sampling rate = n will force your flowgraph to generate only n points per each second. This is used to control the processing speed of GNU Radio when you are not attaching a hardware. (you should not use the throttle block when you are using hardware transmitters/receivers, as this will cause a two-clock problem).
Other than that, the sampling rate variable is just there for conveniences in calculations (so as you can think in Hz, not in -pi to pi).
Regards,
Kyeong Su Shin