HoP can be used in several fashions:
1. It can be a pseudo +buy. If you need a +buy, but one isn't out, HoP can let you much more efficiently build an engine. This is particularly useful if your engine requires multiple components. E.g. village/torturer/chap benefits greatly from a HoP as you will virtually always be able to get a village and pretty easily hit torturer or gold (copper, silver, village, chap, torturer gets you to 5). Being able to pick up both engine components every turn is pretty big, later in the game you can just start spamming gold and province with a late game conversion of HoP -> duchy to break a tie.
2. It can be used as alternate payout. For instance let's say you want a goons engine on a board with hamlet, chap, and menage. Chap, hamlet, menage, copper, silver, and HoP gives you a goons far quicker than going for multiple silvers or golds. Further it can keep on being used for goons allowing you to use your buys for more hamlets/menages. If you hit 5, HoP is a good buy.
3. It can be used for the province megaturn. Ideally you want a good number of unique, cheap cantrips. Pawn, hamlet, menage, pear diver, haven, and peddler all work quite well here. Also some source of +cards is also good - smithy, lab, golem, hunting party, etc. all work well. Pretty much you simply build a nice engine (trashing & sifting are big helps), load it up with the best of the unique cards that aren't already part of your engine, and then cash out can secure a more points that your opponent can before game end.