UK Ultimate is a membership organisation - it is effectively owned by its members who are then able to have a say in how the organisation is set up and run. Membership fees are used to help meet the basic costs of running UK Ultimate.
There are two levels of membership: Full and Basic. A player needs to have a particular level of membership in order to be eligible for certain UK Ultimate events.
In particular you need to have Full Membership to be able to play at UKU Tour events and Nationals (full details here). If you will be playing in a team that is entering these tournaments, or a team that will be playing in a Nationals Finals tournament (e.g. University Nationals) then you will need Full Membership. Don't forget that students and U18 players get a discount by purchasing a uni/U18 full membership. Full members have the option of requesting a printed copy of Ultimatum be sent to them.
Some players only want to play in the major events from time to time and so prefer to buy One-Off Event Eligibility for these events when they occur, rather than committing to the full year up-front. If you only play one event per year, this is cheaper, but otherwise it probably makes sense to purchase the annual membership.
It is a requirement of the organisation's insurance that UK-resident players at UKU events are members of the organisation. We recognise that in some situations the cost of Full Membership would be prohibitive, so we introduced a very low cost alternative - Basic Membership that provides eligibility for a small subset of our events. In order to keep the price as low as possible Basic Membership does not include a copy of Ultimatum! - although it is possible to obtain an Ultimatum! subscription seperately.