Why express protein in bacteria?

Answer 1

You should give some more details.

I guess you are referring to producing proteins which will then be isolated and used in their pure form.
Bacteria can be made to produce large quantities of proteins, even more than 50% of their total protein content can be the desired protein and there are lots of easy ways to purify them from bacteria to an extremely high degree of purity.

Also bacteria are easy to grow, handle and manipulate compared to other organisms and have all the necessary machinery to express the protein effectively and efficiently compared to in vitro synthesis. However some proteins need to be post-translationally modified in ways that only eukaryotic organisms can do that. In such cases you need to use a different expression host like yeast, but still this is not good enough for all proteins.

Expressing heterologous proteins in bacteria for in vivo studies is a whole different story, but I don't know if you are referring to that.

Answer 2

Plasmid or circular DNA found it bacteria is easier to work with. Easy to grow them and very fast. Better than blending a rabbit to get some protein out.