EVERYONE should have a bootable HD clone handy.
Some advantages of a cloned HD:
In case of an internal HD crash and failure, there is absolutely nothing quicker to getting back to 100% operation than having a HD clone handy to either boot from, or within 20 mins. installing and removing the bad HD. Nothing to install software-wise, and a speedy immediate return to your computer use and productivity.
If you do an option key startup at boot you can directly boot to your cloned drive externally and operate from same like normal.
Sandboxing: With the help of the Cloned Drive, you can perform sandboxing, or the testing of new software, their updates and applications before they are installed on your Mac system. Moreover, if there are issues with the system, you can troubleshoot them by booting off the Clone. Many people test new APPS or experiment with a secondary clone and never worry about a failure or serious issue since the internal HD is untouched and the secondary clone can be wiped afterwards if any major issue occurs in testing. This is an invaluable tool in many instances.
If you sell you computer for purchase of a new one, you can take the clone you have on hand and install same or clone it to the new internal drive of your new Mac.
As it turns out one of the huge positive benefits of a clone is that people were seeing faster HD speeds; with APPS booting up from the clone than they had seen with the internal HD. The cloning applications in cloning the drive defragments the data to the clone and remove the “holes” in the cloning process.
With a small portable HD clone you can take your ‘computer’ anywhere to most any other current Mac and boot from your HD clone and have your entire system and its data immediately there for use. In case one is across the world and their macbook gets stolen, or damaged, with another Mac you can be up and 100% in the time it takes to boot to the new or borrowed Mac!
You can clone from your external clone to the internal HD/SSD in case of corruption.
The best thing that can be said, ideally, when your HD crashes with all its data is “so what, I’ve got a clone right here” and you can return to normal operation within seconds (by booting from the clone) or minutes (by swapping drives).