Before we begin, IKEA is great. You can argue about its durability, or when it's worth it at all to buy cheap furniture, but the company doesn't stay in business because it's furntiure build quality and design are awful. That said, it's no panacea. IKEA furniture can fit a lot of shapes and spaces, but it's not the same as raw materials, and often costs more than just getting the raw materials for your DIY project. Sometimes, it's just cheaper and more efficient to go to Home Depot and buy some wood, paint, nails, and go to work.
There are likely some other examples of when IKEA is a good option too, like when what you want is essentially already an IKEA product, it just needs a little work to fit where you want it to go, or do what you want it to do. IKEA Is also a great option is you're just getting into DIY, and, as we mentioned, you're not comfortable with,have access to, or space to use nailguns, table saws, or a few gallons of paint.