They're likely helping to create positive experiences and memories for other people during their travels. Some of my best memories in life are thanks to random solo travelers I've met during various trips.

I met a solo traveler waiting for the airport bus in Thailand, and the next week we did a road trip across the country and back together. I met a solo traveler in Hungary on the train that needed accommodation, so they stayed in my living room and we spent a few days traveling. I met a solo traveler in Italy, and the following week we decided to hitchhike to various coastal cities. Their "selfish pursuit" brought me all sorts of value.