Why do some types of ancient Greek and Roman amphorae have pointed bases?
I thought they all had pointed bases? 
Well, according to this site, the corks were marked with their contents and destination, and they were slid into racks bottom-end first. That’s easier to do if the bottom ends are pointed.
They did, however, make some flat-bottomed; most were pointed.
I understand amphorae, basically, to be storage vessels for wine, grain etc and as such presumably, at some stage in use, it would be convenient to stand them upright. While a vessel with a pointed end might stand upright in (for example) sand it wouldn’t stand on a hard surface without some kind of support. And amphorae with flat bases would slide into a rack just as easily as do present day wine bottles. Any more theories?
I’m pretty sure I’ve seen some contemporary(to the time) paintings or similar showing wine being poured from amphorae - one hand holding a handle at the neck, the other hand holding the pointed base and tipping - whether this was a design ciriterion though I couldn’t say.
But while it seems a daft way to design a " big bottle" there must have been a perfectly good reason for it.
I find it difficult to accept the (horizontal) rack explanation because leakage of liquid is more likely in that position.
I can understand,Mangetout, that it would be easier to tip a large vessel with a pointed end but flat bottoms have many more other advantages.