what are the en pins (1,2EN and 3,4EN) used for on an SN7544 H-Bridge
The page you link to has a function table that says it all:



When an enable input is LOW, the effect is that the motors will be stopped.

The SN754410 datasheet shows how the inputs of that chip are implemented:



This circuit has the property that an unconnected input is high (you must pull it low to make it low). Hence you can leave it unconnected if you don't want to use this input to make the motor brake.

NOTE: most modern chips do NOT have such inputs, and don't like it when their inputs are unconnected. It is good engineering practice to connect all inputs to a well-defined level.