Buy One, Get One Free

Many components come in both single-device and multi-device packages. The 555 timer is available as both a single timer in an 8 pin IC and as the 556, with two timers in a 14 pin IC.


Frequently, these both cost the same price, or so close to it that the difference is negligible. The same is true for op-amps, comparators, and many other devices.

So, if you can afford the extra space on your PCB, stocking the multiple device versions of the parts is more economical, even if you often only use one of the devices. Each time you use the second device, you're getting it practically for free.

Also, with the various uses that most devices can be put to (op-amps can take the place of many logic gates when wired properly, for example), they give you extra "wiggle room" in your circuits for growth or for changes to the design.