CAD is short for Computer-aided design. That you are reading this means you probably already knew that.
But why use CAD? The short explanation is that CAD automates away many precision functions that are necessary for engineering drawings. The longer reasons are so varied that you are likely to get about 200 detailed answers if you ask 100 engineers, designers or architects.
Drawing for engineering purposes is a lot different than drawing for art. In art the concept of scale is arbitrary and often deliberately warped to suit the needs of the particular art. It would be almost impossible to build a correct building or engine from a strictly artistic depiction — and if you did build such a structure the result would likely turn out to be cartoony or unsound.
Architects, engineers and other creators play the role of artist, inventor or problem solver (usually all at once!) at the beginning of a project and get to have the fun of designing pretty spaces or marvellous things. They then must switch hats and have to figure out how to make their concept into a real building, machine or other useful object. Reality brings with it many constraints that art ignores, and this is where CAD suddenly becomes useful.
When you draw inside a CAD program you are not simply drawing, you are also simultaneously plotting every aspect of the projected concept in very fine detail. The precise coordinates of every line, curve, corner, point, and every other object are all knowable from within a CAD drawing, and herein lies one of the chief advantages of CAD over hand-drafting. If you have a desire to build a somewhat abstract-shaped room, for example one with 7 corners and uneven length sides, it may be a bit of a chore to measure out exactly how long each stringer or beam should be in the room prior to cutting. When doing this by hand on a drafting table the lengths are knowable and measurable, but the time and effort required is sometimes not worth the result and the process is somewhat error-prone. On the other hand, if you design such a room within a CAD program (whether you draw some free-hand concept art on paper first or not) the CAD program can tell you the precise dimensions of every part rendered in the drawing instantly and with surprisingly little expert knowledge on the part of the designer.
There are many other advantages to using CAD — such as the ability to instantly share digital schematics in perfect detail with anyone in the world (something not possible without considerable effort when using paper blueprints), speed of editing, low cost of pre-production redesign, the ability to link your CAD schematic to a robotic production system, the ability to using scripting facilities within the program to automate mundane tasks, etc. These advantages will likely become more apparent as you read further in this manual, but if those are uses that don't concern you, don't worry! LibreCAD is easy for beginners to use, free of cost and open source anyway — so unlike proprietary software you can either implement the features you like yourself or get involved with the testing and development of the program and request features that you need.
- Computer-aided design on Wikipedia.