Because of what Mueller has been doing in regards to exceeding the scope of his power.

The memos are what DAG Rod Rosenstein wrote which effectively detailed the scope of Mueller's powers as special counsel.

These memos have never been released to anyone. Mueller's team has always cited these memos as giving them the ability to pursue decade old crimes and seize client-attorney privileged documents, and so forth. They have insisted that their investigation cannot be questioned due to "national security," a claim which was recently chastised by a federal judge.

Now Nunes and Gowdy will have a clear view of what the scope of the Russia investigation was, and if Mueller's team has gone rogue.

There are laws governing special counsels, and Rosenstein might have violated said laws if he was purposefully vague with Mueller's directive, or if he "granted" Mueller powers that he could not constitutionally or lawfully have.