This is the Modern handling of the Steinitz Defense. Black pushes back the bishop before playing /\d6, so that /\b5 will be available, if needed, breaking the pin against the knight at c6. Boris Spassky has been among the advocates of the Black side. It has strong appeal to strategic planners who like to work against weak squares in a methodical positional manner, but sometimes attacking play liven things up.
1.e4
e5
2.Nf3
Nc6
3.Bb5
a6
4.Ba4
d6