In some circumstances, having the blueprints might lead you to later construct a similar building, or convince an insurance company that your current one is safely built. If a builder refused to provide you with blueprints of a building, but expected you to buy it anyway, you might balk at the deal.
With software though, it has become normal to do something very similar to buying a building without the plans. Most people buy software with no way (certainly no legal way) to discover how it's constructed or how to improve upon it by examining the sourcecode -- the software equivalent of a blueprint. When you buy a box of Microsoft Word or Adobe Photoshop (I say "box" because in general, you can't actually buy the software itself, only heavily restricted permission to use it) you may end up with some very powerful software on your computer, but no blueprint.
(If you think you have bought any secret-source software from Microsoft or other large vendor, you might want to read or reread the license -- I'd appreciate knowing if you find an exception.) Not only do you probably not have the right to do very much with the software beyond use it on a single PC and in limited contexts, but the software maker has probably disclaimed responsibility for just about everything which could possibly go wrong as a result of its software, including data loss and equipment damage, and done so with a license they're nearly certain you didn't actually read before purchase. In our litigious society, it's hard to blame them for the legal bluster, but are they treating you as a valued customer, or just a necessary liability?)
Open source software may arrive with few promises, no fancy box, or be less polished than source-secret programs* performing similar tasks, but if it is licensed under any of the common licenses for open-source software (of which there are several, with different degrees of compatibility), it certainly won't make any attempt to deceive the user about his right to use the code. That is, without guaranteeing that it's fit for your purposes or will last through catastrophe, an open source licence at least asserts that you have the right to pore through the plans (the sourcecode), and add improvements as you see fit, and even resell or otherwise distribute the results.
The happy result of the past few decades of source-available software (especially the decade-plus of Linux development) is that raftloads of free, open-source software is available, and that tens of thousands of people at least are enjoying computer systems running sophisticated word processors, web browsers, programming languages, databases, drawing programs and much more, without needing to purchase a single license, and with the freedoms to examine the guts of the sotware they're running and install it on as many machines as they'd like. If that sounds impossibly Utopian, it's not.
It's not only possible, but it's already arrived -- on Earth, right now.