Soften butter and eggs in pie crust?

[Merar]

Ok I know tarts and crusts must call for cold butter, I read about kinds of pies such as the sablee or the brisee but I noticed some sweet tarts call for eggs in the crust, and others call for soften butter (room-temp)
What characteristics does that give to the dough and in what kind of pies does it require to have either one of them???

 

[bakechef]

Cold butter is usually used for flakiness. Using softened butter and an egg would produce a more shortbread cookie type of crust, more crumbly than flaky like a typical pie crust.