At work I simply use the shortcuts "M" to mark as read and "A" to archive and almost everything is archived, except for useless information. I also have lots of filters configured, which put emails into appropriate local subfolders, so that I can access them already knowing their category and mentally parse them faster.
In private, I have a much more elaborate setup, making use of saved searches and sub saved searches, tags, multiple e-mail accounts. Private e-mails have much more variety than work e-mails and I also do not see the need to keep them all. However, I still keep many, for example for all kinds of programming languages in my kind of "read later" subfolders. If I was to look at all of them, however, I would probably have years of work ahead of me. It is probably just a way to categorize and declutter the inbox. Every few weeks I need to be really determined to get my inbox to below 10 e-mails, otherwise uncategorized e-mails will fill it up quickly. Sometimes I need to revise my folder structure or, rarer, need to add another saved search.
I always find my e-mails either by looking in the correct folder, saved search or by using the global search or view local search.
I find Thunderbird indispensable for my workflow and highly configurable. The only thing lacking is a compound condition system for saved searches or filters instead of only having disjunctive or conjunctive single attribute conditions.