Platforms track which IP addresses log into which accounts, and multiple unrelated accounts logging in from the same address is a common signal used to detect coordinated or fake activity.
Why this matters for managers
Agencies and marketers who legitimately manage several separate client accounts need those accounts to look independent from each other, which is where dedicating a distinct proxy IP per account comes in.
Why free proxies are risky here
Public proxies are shared with strangers and often already flagged by major platforms, so using one for account logins can trigger security holds or verification challenges rather than helping — this is a case where a dedicated proxy matters more than saving money.
A word of caution
Always follow the platform's actual terms of service. Using proxies to manage accounts you're authorized to manage is very different from using them to create large numbers of fake accounts, which most platforms explicitly prohibit and actively detect for.