One purpose of grouting (I prefer that term to "cementing") the beam end in a beam pocket is to resist rotational movement. Rotation is more of a concern when the beam is tall and narrow and less of a concern when the beam is the same width as the height.

If the beam is tall and narrow AND is very long AND does not have lateral support along its length it is more likely to rotate. Having rotational restraint(s) at the end(s) of the beam would be important in that situation. Properly placed grout will restrain the ends of the beam from rotation as the rotating beam will be placing the grout into compression against the sides of the pocket and grout, like concrete, is very good in compression (but not tension).

I don't think the fire protection provided by grouting the beam in the beam pocket makes any difference whatsoever. Assuming a simple beam (no rigid end connections), the stresses at the ends of the beam are very small compared to at mid span. If there is a fire hot enough to cause the steel beam to lose strength it will fail at mid span long before it fails at the end supports.

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