You don't understand the process of mining. Every mining device that participates in the search for a valid block header adds to the global hashrate. Why? Because finding a valid block header is a random process. Any hashwork of any miner, regardless if solo or pool, draws lottery tickets to find a valid block header. Double the number of miners or their hashes/s and you double the number of drawn lottery tickets. It doesn't matter where they draw their lottery tickets.

It's crucial to understand this and I have doubts you do.