I suppose much comes from MadMalkavian stating s/he's living on a somewhat tight budget.
You can live cheaply and have nice things, but that may require working on certain skills, with corresponding risks.
Can't afford ruining meals as I learn to cook -> keep buying overpriced junk food
Can't afford ruining something building my own PC -> keep buying overpriced weirdly put-together computers, to be replaced when a component fails/becomes obsolete.
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For computers, this used to be a no-brainer, but no more. The average off-the-shelf PC is ok-ish value for money these days, building your own is most interesting for people who know exactly what they want to spend their money on.
Some examples: Pre-assembled near-silent builds tend to be priced as premium products, if that matters to you building your own is much cheaper. If you care about a nice responsive system in everyday use instead of raw power, spring for a Solid State Drive... doesn't apply much to gamers on a budget. You may have obscure needs (Dwarf Fortress is limited by a fairly odd metric - RAM latency. If you want to run non-Windows operating systems, you may pick hardware based on how well it's supported there).
Of course, much of the time you can satisfy many of your needs in a pre-assembled build. I'd advise against Alienware though - they are still living on the reputation from a bygone era.
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Last but not least: Even if you don't build your own, get comfortable replacing some components. More RAM is often a cheap high-impact upgrade, graphics cards obsolete much more quickly than CPUs.