lit. one doesn't visit a temple without a cause (idiom)
fig. to visit sb with an ulterior motive (esp. to ask for sth)
Definitions (粵典–words.hk)
語句
有事相求先拜訪 to visit someone only when asking him/her a favour
無事不登三寶殿，佢為咗封推薦信先嚟揾你。
无事不登三宝殿，佢为咗封推荐信先嚟揾你。
mou4 si6 bat1 dang1 saam1 bou2 din6, keoi5 wai6 zo2 fung1 teoi1 zin3 seon3 sin1 lai4 wan2 nei5. He finds you only when he needs a recommendation letter and asks you a favour.
Definitions (Wiktionary)
phrase, figuratively, idiomatic
One would not go to someone's place except on business, for help, etc. / I wouldn't come to you if I hadn't something to ask of you.