The term "Social Engineer" was popularised by Kevin Mitnick in the early 90's however, these techniques have been around for as long as humans have been.
Generally speaking, social engineering attacks are the platform for the majority of breaches. According to a report by Cofense, social engineering scams stole over £4 billion worldwide from 2013-2016 cofense
Data suggests that only about 3% of malware attempts to exploit an exclusively technical flaw. The other 97% instead targets people directly through social engineering.
In some cases, attacks are meticulously planned to gain access to the information they want for any host of nefarious reasons. Creating complex pretexts involving full social media personas, with backstories and friends they communicate with. Simply to gain access to some information that would seem trivial at first, such as an employee's email signature.