Recently I started learning blockchain and related technologies.
There is SO MUCH to learn.
I started with blockchain basics (and it inevitably uses bitcoin as an example)
If you are new to blockchain (not ZERO knowledge) I recommend you checkout the blockchain demo
It is one thing to say you can not modify "old" blocks, but visualizing it really makes the difference. (It is important to understand it for the topic under discussion)
Anyway, back to "uncles".
If you know how bitcoin mining works, you know that "orphan" blocks are discarded. (i.e. yours was not the longest chain)
They also lead to "wasted" efforts, since miner does not get any reward for such blocks.
But ethereum is different.
In ethereum, such blocks are called "uncles", and it "encourages" the miners to include the uncle blocks. (and yes, miners are rewarded for uncles)
The only reason (that I understood) is that including these blocks makes the chain "heavy", and thus more secure.
(If you haven't understood why/how it becomes secure, please watch the blockchain demo again. See the link above.)
On a lighter note, but still relevant.
Someone on reddit explained these blocks as :
They are same level as your "Dad" (parent block)
They were "almost" going to be your "Dad", but didn't. Your "Dad" beat them. (i.e. Your parent block was the "longest chain", and thus "won".)