Something like 80% of hacks start from some sort of a phishing attack.
Sending malicious excel/word files was a very large avenue for this for years!
DDE (dynamic data exchange) essentially allowed for remote code execution on the machine, once the document was opened.
In security class, as a junior in college, I created a demonstration of this using a Word file that would download my Github repo from the internet that would attempt to find the string "11". Once this string was found, it would replace it with "pwned". Essentially, it was a keylogger, waiting for a particular string. I was trying to mimic the bitcoin address switching malware.
Although Microsoft has locked down on this for a couple of years, people occasionally find bypasses that make this exploitable again.