Usually, people are interested in expressing human proteins in fast-growing organisms like bacteria to yield a maximum of protein within a short time lapse. Codon usage harmonization/optimization have been used to increase the protein production. The role of codon usage have been studied in the context of viral protein expression, but outside this viral context, is there any example of using human cell lines for heterologous expression?
Besides research (as mentioned by mxwsn), there are a lot of actual protein based medicines and vaccines produced in human cell lines. One of the first mass cultures of a human cell line was in the monkey free production of a polio vaccine (source). Almost every researcher working in a molecular lab has grown HeLa cells at one point during their career!
Expressing GFP in human cell lines for research purposes is an extremely common example of heterologous protein expression.
For research purposes, it's usually unnecessary to optimize codons for increased expression, especially since using more efficient enhancers and promoters produces a far greater improvement in expression levels than codon optimization which is non-trivial and species-specific.