I'm confused as to why they might implement this next year. What are the pros of an 18-game season? What is the reasoning behind it?
It comes down to money and revenue.
Now you might ask if more games bring in more money for the NFL, how come the number of games hadn't increased years ago? That's because of the current CBA signed by the league and the NFLPA in 2020 allows for it. The agreement allowed for an extra regular season game by reducing a preseason game.
So now, we must ask, why would the NFLPA agree for an extra game, when the players don't get paid more as their contracts remain the same? Money is the answer here too. It's because the players would get a bigger share of the NFL revenue once the 17th game was to be added. For the extra game to kick in the NFL had to sign more media contracts and raise players' revenue share. The extra media contracts were part of the agreement to ensure higher revenue.
With both the league and players financially gaining from the deal, it was just a matter of time for the 17th game to be added once the CBA was agreed upon. Covid-19 had a big impact on the league's revenue, and also lead to a decreased salary cap for the 2021 season. This contrasts a yearly raise of $10 million or in the recent years. So in 2021, the league signed more media contracts, which allowed it to add the 17th regular season game, helping itself and players earn more, and teams to have higher salary cap in the coming years.
It's also noteworthy that the extra game will result in revenue from the sales of tickets, venue food, merchandise, etc, as well as new inter conference rivalries.
It's all about money: teams make money from fans buying tickets, from fans paying for parking, from fans buying beer, from fans buying hotdogs, from fans buying nachos, from fans doing everything else fans do on game days at the stadium. 17 games gives them 6% more game day income than 16 games. 18 games gives them 12% more game day income.
More games also makes for a more attractive package for the NFL's television (and other media) partners, so they can sell the rights for more.