What is it exactly that they do and why do I need them?

In simple terms, The Trust Store contains the digital signatures of issuers of the SSL certificates that are used to encrypt access to websites and email servers. There are billions of SSL certificates in use around the world. They are issued by a relatively small number of agencies and companies (called Certificate Authorities, or CAs), who, as part of the process of issuing the certificate, verifies that the site is legitimate. When you visit a website the certificate from that site is sent to your browser. Your browser then verifies that the certificate was issued by one of the agencies whose signature is in the Trust Store. If the signature is not in the trust store you see a warning that the site may not be what it seems. The Trust Store is updated as part of an iOS update. It will change whenever the list of certificate authorities changes, which is very rare. The Trust Store version is actually the date and time the trust store contents were last updated. So, for example, version 2023032800 was last updated on March 28, 2023 at midnight.