Like some of the other answers have said, every account has at least a small part of your user information in it somewhere. While that account exists, there is a possibility of that information leaking out through:
An attacker accessing the account: Someone may guess your password, or it may become part of a leak and an attacker will log in and copy out your information.
The site leaks your information directly: Even without exposing the password, the provider may leak your account information as part of an attack, or through privacy-hostile actions.
Deleting your account is one good way to avoid this kind of information leakage.
But I would argue that deleting the account does not protect you from the second case. When you delete your account, nothing (except recently GDPR in the EU) requires the provider to delete your information. So simply deleting the account cannot protect you completely.
Instead, I would recommend inserting new, false, information into all parts of the account (changing your address, real name, email, etc) then deleting the account. It still cannot fully protect you, but it may prevent the information from being leaked in the case that the provider keeps you information around.
TL;DR: Yes. Deleting your accounts is a good idea for security. But more can be done.