Concomitant comes from late Latin concomitant- ‘accompanying’, from concomitari, from con- ‘together with’ + comitari, from Latin comes ‘companion’.  So something that is concomitant is like the companion of the main event.

According to the Oxford English Dictionary, the word "concomitant" was first used in English in the early 1600s, specifically in 1608.  The OED's earliest evidence comes from the writings of Edward Topsell, a Church of England clergyman.

Examples of concomitant used in a sentence:  
"It has been argued that sputum eosinophilia is related to concomitant features of asthma."  
"Nor have changes in policy and orientation been accompanied by concomitant changes in legislation." 
"As an old politician, he has picked up all the concomitant features." 
A right-to left shunt at the atrial level was present, and concomitant cardiovascular malformations were excluded." 
"Other outcomes include increased incidence of chemical use, with its concomitant risks to humans, livestock, a nd natural systems." 
"Such gas is a more or less general concomitant of oil all through the petroleum-bearing areas of the country." 
"Sedation due to the drug may be increased by the concomitant use of other central nervous system depressants." 
"Corruption is the frequent concomitant of privilege, and thus the town councils often connived for a price at the presence in their midst of Jews whose admission was illegal." 
 

Just the facts:

CONCOMITANT Definition & Meaning - Merriam-Webster

CONCOMITANT Synonyms: 52 Similar and Opposite Words | Merriam-Webster Thesaurus