wrote

I was curious why there are iron pipe fitiings and compression
fittings?

Historically heating pipework was run in galvanised pipe with threaded iron
fittings.
In this case the joint seal was provided by sealant (boss white and hemp
originally) wrapped around the male thread before the joint was made.
Compression fittingsAFAIK came into popularity with the use of softer copper
pipe and rely on the crimping of an olive to the outside of the pipe to form
part of the seal. The other part of the seal is made between a taper on the
end of the olive and a corresponding taper on the fitting.
At joints between fixed equipment like hot water cylinders, there is still a
requirement to make a thread seal (usually nowadays achieved using PTFE
tape) before conversion to compression fittings or soldered.

But with the advent of plastic, a lot has changed. As well as threaded and
soldered copper work close to equipment (plastic is not recommended close to
a boiler for instance) there are push fit fittings. To confuse things,
compression fittings can be used on plastic but require a plastic
reinforcing sleeve be inserted in the pipe end.

The plastic option has a great deal of support in this NG partly because of
speed and ease of install.
Many it has to be said remain to be convinced.

HTH


Phil