Why do some pies call for two crusts, and some do not?
Yes, it is mainly tradition, appearance, and how you like the ratio of crust to fruit in your pie. You can make any pie open faced, with a full top, or any type of lattice top. The other thing to consider is that a top crust provides some heat insulation to the fruit, so if you don't want the fruit to caramelize as much, it will help with that.