In theory there is less likelehood of warpage and less groove noise (due more to the purity of the vinyl used in production).
A lot of older LPs (1950s - early 1960s jazz & classical especially) were quite heavy anyway, and pressed on harder plastic. Later on, LPs were made thinner and more recycled material was used in production. This was due to various factors like the explosion in demand caused by pop & rock (and a huge 'baby boomer' customer base), and things like the oil crisis of the early 1970s quadrupling the cost of raw materials).
In the 1970s companies like JVC were experimenting with quadrophonic formats (like CD-4) and the ultrasonic signals necessary for processing quadrophonic (with special styli) could literally be worn off a pressing made from the typical grade of vinyl in use by then. So some of these Japanese companies started using heavy, hard vinyl to preserve the quadrophonic signals in the pressing.
Imported Japanese re-pressings on this grade of vinyl became popular with enthusiasts even after all the quadrophonic formats were dropped.