There's nothing wrong with Wal-Mart - it's not even the cheapest place for groceries.
Real Canadian Superstore and No Frills are often cheaper for groceries
http://flyers.smartcanucks.ca/canada/walmart-west-flyer-april-16-to-22/all
It falls into the same general category as Zellers, Woolco, K-Mart and other "junior" department stores (as they were once known) of yesteryear.
You'd buy staples there - drug store items, household items, hardware, automotive items, but not necessarily clothing.
A Wal-Mart would drive a lot of traffic into Brentwood, esp. from Vancouver with SkyTrain access just past Boundary (versus all the way to Lougheed or Guildford).
If Sears remains, Wal-Mart and Sears are pretty compatible anchors. If Winners relocates from Madison Centre (maybe it can become a Marshalls and a Winners/Homesense could open at Brentwood) at the west end of Brentwood mall, it would be a good set of anchors for the mall.