Asphalt underlayment

[Heaviside1925]

Hey Civils! So, I do a bit of interstate driving and today I was passing a construction zone where they were rebuilding an entire lane. They had demo'd down to the base, where they were adding drainage, regrading and pouring a concrete mat with an asphalt topped road surface. All of this seemed typical to what I see on most all projects like this. The one peculiar thing I noticed though is they had installed a layer an asphalt on top of the base and were setting the chairs and bar on top of the asphalt prior to placing the concrete. Is this a standard construction method? Is it done to precisely control the depth of the concrete? Are there other engineering considerations for doing this?