With the rise of ASIC chips and ever more powerful GPU cards solo mining has become increasingly less profitable, as the difficulty of mining a block has increased to the point that it is highly unlikely a solo miner will find a block on their own.
This has given rise to collective mining groups. Initially individuals came together to mine collectively, but soon the business opportunity of this became apparent and now we have companies investing in high-end hardware in order to lease hash power to individuals.
It also eliminates the need to maintain expensive hardware, and deal with the heat and noise generated by modern high-end mining hardware. Collective mining spits the blockchain rewards generated with all of the miners in the collective.