Using aliases for certain apps/services will segregate that account from the primary email address and/or account. If the email is somehow revealed there is little risk. The account has no login capabilities (as it is only an email alias) and thus any brute force login attempts will be a waste of time for the attacker.
Conversely it works well as a honeypot since any login attempts to your primary email provider using an email alias indicates nefarious activity.
Example: suppose my primary username and email is
email@example.com. An alias gets created for an online service
OnlineService cannot login to
Having unique aliases can also help to identify spam/phishing. Suppose an alias is created for your
eBay account and an email regarding insurance payment is recieved. If the primary account was used for all external services perhaps the email would have been read. However something regarding insurance delivered to your
eBay alias allows for it to be quickly deleted.
Data Breach with Unique Credentials
Any breach would only impact the service where the breach occurred. For example, if
GMail is being used and a data breach hits
eBay, then only the logon credentials used for
eBay would need to be changed. Really only the password would need to be changed.
Data Breach with Shared Credentials
If logon credentials are being shared across services then a data breach in one app/service would impact any app/service using those same credentials. This is why credential sharing is bad.
The downside of the approach you're inquiring about is keeping track of all the aliases and associated passwords. Consider using a password manager or an offline method like a notebook to manage this.