You don't - similarly you don't need a Mac Book to run OS X (technically at least); You buy the full-flag NAS or MacBook because it's a bundled supportable quantity that reduces your cognitive load in exchange for money. Synology for an SMB or home lab is pretty good stuff and you don't spend (as much) of your time editing smb.conf, or configuring the core backup services or whatever. Some clickops and you're done - and you can do a lot - under the hood it's still Linux (or at least mine is), you can SSH in and do damage -- the hardware isn't "special", it's not necessarily substantially better than another system that could handle a similar number of drives in any measurable way.

I have a synology because I got tired of running RAID on my personal linux machines (had a drobo before that for the same reasons) - but as things like drive locking occur and arguably better OSS platforms available, I'm not sure I'd make the same decision today.