Short question: why should I purchase Substance Designer?
Short answer: because it'll save you infinite amounts of time in the long run, and time is money.
I'll admit I was hesitant to purchase SD; it looked to me as though I was paying $75 for an edge wear generator, so I held back for many months until it went on sale, but now that I've purchased it, I cannot imagine my production pipeline without it. Creating a substance may take me a few more hours than creating, say, a photoshoped material, but when done, I can alter that substance in so many ways SO incredibly fast that I can create realistically about 20 to 50 completely different materials in the time it'd take me to create three or four non-substance materials from scratch. And remember, these are procedurally generated and entirely seamless materials that take up literally under HALF A MEGABYTE in file size.
For anyone who uses non-Substance enabled programs (like me [goooOOO BLENDER!]), don't let that stop you from purchasing SD. Yes, you will have export out your substance's bitmaps and reapply them every time you want to change it, but that takes three minutes max; if you had to create a new material every time you wanted something different without SD, I guarantee you it's going to take longer than three minutes.
Whether I'm using SD to create procedural materials or using it to fine tune my models' bitmaps, I have found the enitire experience worth every cent I spent on it. For those of you who are interested in creating seamless, tileable materials, I "highly" recommend SD as this is, in my opinion, it's strongest front.