Why to calculate the current gain in transistor?

A: Transistors are really voltage amplifiers since the effect is voltage across a load. Using beta as a means to calculate gain is an approximation. basically is the ratio emitter resistor and collector resistor with no load

No. Transistors (bipolar junction types, what most people think of as "transistor") are considered to be current devices.

Although it's possible to specify a transconductance (output current/input voltage), this has not been done since the earliest days of transistor theory, and you will have trouble actually finding a transconductance specification in data sheets.

Current gain is (output current/input current).

In a common emitter circuit, this is (collector current/base current), known as hfe or "beta", with ranges from as low as 10 for high-current devices to high hundreds for low-power audio types.

In common base, it's (collector current/emitter current), and it's just a bit less than 1.0, typically 0.95 to 0.999. This is known as hfb or "alpha".

In common collector, it's (base current/emitter current), known as hfc, with about the same values as hfe.

The exceptions are the obsolete point-contact types of the 1950s/early 1960s, and the various types of field-effect/MOS devices known variously as FETs, JFETs, IGFETs and MOSFETs.

The field-effect/MOS devices have very low/zero input current, so they are not specified for current gains.