Why people sending btc to this address?
Bitcoins sent to these addresses are gone forever. There are a few reasons whey people might send to a burn address. Primarily, sending to a burn address proves that you spent the bitcoins and got nothing in return. Some people send just like some people might burn a $100 for show in a video. Many of the bitcoins sent to 1111...4oLvT2 were sent accidentally due to software bugs.
Why it called burn wallet?
It is called a "burn address" because bitcoins sent to it can never be recovered. They have been "burned".
What is burn wallet?
A burn address is an address with an unknown private key. It is not possible to prove that the private key for a burn address is not known, but we can be pretty certain that the private keys for these addresses are not known because addresses that have private keys look random and those addresses are far from random.
Burn addresses are easy to construct. In short, you set all but the last 6 characters in the address to the text that you want and then compute the last 6 characters as the checksum.
The person that owns the address used vanity address generator to generate the address with high computational power equipment. If the person generated it with the right vanity address generator software, the address is very safe just like the normal bitcoin addresses.
Burn addresses such at those cannot be made with a vanity address generator because every character in the address (except the checksum) is chosen instead of generated from the private key.