Why do I need to wear a condom if a prostitute gives me a blow job?

Safe for the prostitute

If you wear a condom, the prostitute does not risk an STI. If you come while having a blow job with a condom, there is also no risk of hiv for the prostitute.
The condom prevents occupational hazards for the prostitute.
So, out of respect for the woman and her profession, always use a condom for blow jobs.

Safe for you, the client

If you wear a condom, you do not run a risk of contracting an STI if you are given a blow job. Without a condom, you run the risk of contracting herpes, syphilis, chlamydia and gonorrhoea.
Even if the prostitute does not obviously have a cold sore or syphilis sore, you can still contract herpes or syphilis on your penis.
If the prostitute performs deep throat on you, you can contract chlamydia and gonorrhoea if the prostitute has these bacteria in her throat.