While another comment covered the matter of security updates, another point that is safer about repositories is the security of the binary and the transaction. Meaning, the place you get your software from and how this transference is accomplished are also security sensitive.

When you get the software from a repository, you typically have some assurance that (a) the binary you are getting was compiled from the source that is published (b) the source from which you are downloading is known and trusted © the method through which you are transferring is somewhat secure (e.g. TLS) (d) the changes made were inspected by at least one more independent party (depends on the repository’s policies).

Repositories will also have criteria for inclusion, which can bar you from software you want, but still could also mean software with bad security practices never reached you to begin with. Being included in the repository might also mean it’s up to more scrutiny, as it may be removed depending on what security events happen in the future.

Say that instead we were to get the software directly from the original source. How will this source transfer the software to you? If they publish it on a website, that adds one more attack surface where, if an attacker tampers with files, hashes and/or links, you are now no longer getting it from the source. Say instead you get it from a Git forge such as e.g. GitHub. Is the binary being built form source in CI? Or is it uploaded manually? Does it provide a hash? How can we know the manually uploaded binary, even if it has a hash, was compiled from the publicly available source? There is no trusted, independent third party involved to confirm that.

I can think of a few other reasons unrelated to security, such as repositories, particularly distro/OS-specific repositories but not only them, will tailor the software to your OS, resolve dependencies for you and add niceties such as init system integration, shell completions, man pages and sample configuration that is specific to your OS.