The Temple of Moses is considered to be one of the most sacred places to the Samaritans. They believe that Moses (PBUH) built it in Sinai and Joshua moved it to Gerizim.
The Altar of Isaac is the location where Abraham (PBUH) was about to slaughter his son Isaac to offer him to Allah (according to the Samaritan narration); however, Allah ransomed him with a great sacrifice (ram).
These two places are sacred; they are the directions of their pilgrimage as they perform pilgrimage three times a year: The Passover, the Weeks or Pentecost, and the Tabernacles or Tents.