I've got
a meager 3.245 TB, but I make up for the scant amount by keeping around 75% of it watched. Of course, that doesn't include a substantial collection of raws or my
legitimate DVD collection. Though I suppose if I collected HD versions more often, that TB count would climb; heck, Dantalian no Shoka alone could've taken up 40 times as much space if I'd gotten Tenshi's 1080p instead of m33w's 480p.
This concept of keeping watched movies/anime is so foreign to me. Firstly, i'd probably never re-watch something, and if i wanted to i could always download it again, cause hey, i pirated it anyway. And you're keeping the ones that you didn't really enjoy? I could understand keeping the masterpieces that might have some re-watch value in 5 years from your first viewing when you forgot most of it, but keeping random slap-stick ecchi crap sounds just preposterous.
Hey, some of us
like random slapstick ecchi

, and it's got plenty of rewatch value if you first view it in TV-censored form. And as Mistgun_Zero also said, re-downloading isn't always that easy. It could mean a major hit to your BakaBT ratio, or to ratios on other private trackers (for Funi/NISA stuff), or require you to spend significant amounts of time redownloading from DDL sites (which may be targeted by The Man), XDCC bots or (let's hope not) fileservers. And if nobody kept stuff, who would reseed and reshare files, particularly when some torrent trackers die or get taken down? If people who were downloading in the late 90s/early 00s hadn't held on to their files they got from Usenet/newsgroups/IRC/Limewire/Kazaa (often spending practically a day
per episode), sites like BakaBT wouldn't have taken off so quickly and successfully, or even at all.
And I think people who were around in that era tend to have more of a hoarding mentality, since lost/deleted files would've taken a long time to replace unless they burned them on CD-Rs. It's like the Depression mentality among older generations that grew up in the 1930s -- they save everything that might be useful later, even though times are(/were?) better now than then. I wasn't downloading anime back then, but I still remember back in 2005 when it would take a week or more to DL a less-popular 26-episode series via torrent.
In my case, since I collect actual physical purchased DVDs, that same collectors' mentality carries over to downloaded files. Plus, I tend to collect rare or less-preferred files, like LQ/MQ TV-fansubs when R1 DVD-rips are available, less-popular but sentimentally-valued "out-competed" fansub releases, or SD versions of shows with HD releases available. Maybe I'm catering to a dwindling niche, but someone's got to keep that flame alive.