What is the use of dust bag in a handheld blower(leave blower)?

[Rarestofrare]

I saw a handheld electric blower on a shopping site that says it has dust bag. This is not a hair dryer. Since it just blows away dust, what is the use of dust bag? How can one use the dust bag?

 

[Jrbott]

Search homedepot.com for this one: BLACK & DECKER
230 mph 385 CFM Electric 12 Amp Blower Vacuum

Note it is both, a blower and a vacuum.

 

[callipygenous]

Its primary use is to keep the impeller and impeller bearing clean. There's really no other reason that I know of on a pure blower. Sometimes blowers are reversible and can become vacuums with a catch bag, but that isn't the case here.