delete old, expired transients from WordPress wp_options table
According to Codex, transients are:
a simple and standardized way of storing cached data in the database temporarily by giving it a custom name and a timeframe after which it will expire and be deleted.
NB: by default they are stored in the database, but if you have an object cache like memcached they won't be.
With the current way that the transients API works, expired transients are only deleted when they are accessed after their expiration date. When transients are user-specific or otherwise fairly unique, they can sit there in the database forever unless some housekeeping task is run to clean them up. WordPress doesn't currently have such a housekeeping task. That's what this plugin does.
Only if you have a plugin that is really badly written. Transients can be deleted for a variety of reasons, because by definition they are considered ephemeral. They are considered safe to delete at any time because they are supposedly only ever going to contain information that can be rebuilt.
There are some notable exceptions, e.g. some shopping carts store cart sessions in transients; this is obviously not information that can be easily rebuilt. That data will only be deleted by this plugin if it has expired, which means it would be deleted by WordPress anyway, so it is safe to use this plugin with shopping carts.
On the Tools menu in the WordPress admin, you will find a screen for deleting transients. It tells you how many expired transients there are in your database.
NB: after you install and activate this plugin, the first thing it does is schedule a housekeeping task to delete expired transients. This means that there may not be any transients found when you visit this page in the tools menu straight after installing the plugin, because they may have already been deleted. You probably never need to delete expired transients manually, because they'll be automatically deleted daily.
No. Object caches are limited pools of data, and they already purge old data periodically so that they can fit newer data. This means that old transients will be removed from the cache automatically. It also means that new, fresh transients can be removed at any time too, which is why you should never store anything in a transient that can't be rebuilt easily. See this article on the WPEngine blog for more details: A Technical Transients Treatise.
Not yet. I'll consider adding a setting for that if it seems to be popular. I reckon daily is probably often enough even for busy websites.