It isn't really a past tense but a past participle, and the "past" in "past participle" is really a misnomer. It could better be called a "completed participle". You can say, for instance, "I will have a boiled egg". The egg may well be boiled in the past, present, or future. All you are saying is that in the future, when you have it, the boiling will be complete. Whereas if you say "I boiled the egg", that is the past tense: you are clearly saying this is something that happened in the past.