Another actual answer:
"Seasons" or whatever other word you would like to use for it, are nicely packaged usually-larger content drops that provide benefits for both the players and the business.
From a player point of view, people get new content or a spin on existing content, along with persistence systems that help keep players engaged with a game for longer periods of time. This content usually follows a particular theme that a game / service focuses on for a period of time, before moving on to provide something new and fresh.
From a business point of view, it provides an opportunity for a refreshed marketing beat, which leads to increased player engagement, retention, and better money making opportunities. It also provides employees with more predictable and healthier release cycle, as content can be created and delivered, but gated and made available over time as a season goes on, reducing the dependency on having everything ready at a certain time, and with that reducing crunch-related work behaviours.
The actual provided content can be acquired in many different ways, and business models vary between games.
* You could have expansion-like packed DLCs with their own systems and marketing beats, that require players to pay in to participate, like with for example, Battlefield 1.
* You could have both free and paid content mixed together, with mostly gameplay-affecting items being free to acquire over time, through participating and engaging with the related persistence systems, with additional vanity content available for purchase, like with for example, Battlefield V.
