Stacks often provide redundancy similar to chassis switches that provide redundancy.
For example, a 4507R (the R being for redundancy), supports two supervisors. A stack of two (or more) stack members, generally any stack member can run the stack.
A 4507R supports two power supplies. Often a stack supports multiple power modules so loss of one does not take down any, or more than one, stack member.
A 4507R supports multiple line cards, which supports Etherchannel across them. Many stacks support Etherchannel across multiple stack members.
A 4507R supports routed ports from different line cards. Many L3 stacks support routed ports across different stack members.
A 4507R is managed and configured as one device. A stack is generally managed and configured as one device.
Often a stack supports some higher bandwidth connections between stack members, but it may not offer as much bandwidth as a chassis fabric nor does it operate like a fabric.
Basically, operationally a chassis or stack can be used or configured alike.
Visitor68 notes stacks don't provide redundancy, but as they provide redundancy like some "redundancy" chassis switches, I would assume Visitor68 doesn't consider those providing redundancy either. (I would be curious what Visitor68 actually considers redundancy is. Personally, I consider redundancy when you have hardware that's only really needed to keep the network running when other hardware fails.)
Visitor68 also notes stacks have little value, but my experience, both with stacks and chassis switches that support redundancy, they've kept a network running that without their redundancy would have failed. However, stacks and chassis redundancy aren't the only ways to provide network redundancy, often multiple devices can be used, but Etherchannel is often a nice option of stacks, single chassis, or VSS pairs. (The latter, VSS, is "stack" of two large chassis switches.)
