As part of the build of my amp I was “forced” to investigate and understand the differences between a normal guitar lead (connecting instrument to amp) and a speaker lead (connecting amp to speaker). I suspect that, like many guitarists, I’d just use whatever came to hand, which is usually a guitar lead.
I think most of the confusion is caused by the fact that they both use the same connector – the ubiquitous 1/4″ mono jack plug/socket. If from the start different connectors had been used then it just wouldn’t be an issue.
At the heart of it is that the signal between guitar and amp is very different to that between amp and speaker.
The signal to the amp is a very low current and voltage when compared to the signal from the amp.
The former is susceptible to the noise of external interference (mobile phones and, no doubt, your local taxi company). Whereas the “pumped up” signal leaving the amp has no such problem. The shielding to reduce noise can increase the capacitance of the cable which, in turn, will reduce the treble being passed through.
Cable resistance is nowhere near as significant when dealing with the low power signal from the guitar. When you start pumping more through then this becomes significant and can lead to cable cores heating up and, in extremis, melting.
To summarise, the guitar cable needs to be well shielded to protect against noise and low capacitance to protect against treble loss. The speaker lead needs to be low resistance to cope with the extra power passing through.
Bottom line is that you do need different cables for these different jobs and you should understand why. For more reading, have a look at this article.