This might be easiest to explain by looking at the test in https://github.com/piwik/piwik/pull/10502/files#diff-61dc9c2c1c7174d2576a20aa38f753b9R620

There might be plugin calls like

_paq.push(['MyCustomPlugin::myCustomStaticMethod']);
_paq.push(['MyCustomPlugin::myCustomStaticMethod2']);
_paq.push(['MyCustomPlugin.myCustomMethod']);

and it would not execute these calls if it first loads piwik.js and then later the tracker plugin (only if piwik.js is not writable and the plugin tracker was loaded separately).

To make sure those methods get still executed a tracker plugin can call Piwik.retryMissedPluginCalls(); as soon as it is loaded so these methods will be actually executed and not "missed". If there's a better name for that method let me know. I was thinking about Piwik.setPluginLoaded or something like this but not sure about that as it's not always needed to retry in all cases and as long as it's not official API I rather have a concrete method name for that so it is clear what it does