Is there a reason would you not want to use the first option (5G/4G/3G/2G)?
Yes.
The area I live in London, has had a partial 5G roll out. Meaning that 5g is very good outdoors, but very poor indoors. The phone will always hang on to 5g when at home even though the 4g is much stronger.
When at home I would like the option of switching to 4g for better data speeds.
Just use your wifi you say?! That would be great except Virgin media is down all the time and I often have to rely on my mobile data to fill the gaps.