A test for a venereal disease is recommended:

If there are any symptoms (e.g. malodorous urethral, vaginal or anal discharge, pain when urinating, itching, cold sore or eczema in the genital or anal area)
If your sexual partner is infected with a venereal disease
For people with frequently alternating sex partners
For people who are being treated with a HIV pre-exposure prophylaxis (PrEP)
Before or during pregnancy in order to avoid transmission of the disease from untreated mother to child during pregnancy or birth