To reform the Senate: Why term limits?

Let’s set aside, for now, the inherent ridiculousness of Bert Brown chiding fellow Conservative members of the Senate -- intended as a chamber of sober second thought, and at least nominally a check on the House of Commons -- for their lack of loyalty to the prime minister.  Here’s something I’ve never really understood about Harper’s bid to implement a term limit of eight (now nine) years for appointed Senators:

What problem with how the Senate is currently constituted and functions is this designed to solve?