Excellent question and an interesting comparison. Both Tom and Gatsby clung tightly to the images they had created for themselves. Both men wanted to be appreciated/loved/revered for some depth of character that neither actually had.
Tom and his guests dropped in unexpectedly, primarily (I am certain) at Tom's direction. The purpose of the visit was to demonstrate Tom's superiority over Gatsby by showing off his wealth (Tom mentioned his stables multiple times in the book. Owning multiple horses was a sign of high society.)
Gatsby's parties were an ostentatious display of his wealth, acquired primarily to woo Daisy. Tom already "had' Daisy, and needed to prove that he was more worthy of her than Gatsby could ever hope to be.
Tom's connections to high society mirror Gatsby's "gonnection" to the black market society. In many ways the men are mirror images of each other, with one distinct difference: Tom (shallow and superficial) actually was who he claimed to be. Gatsby, on the other hand, was a total fabrication.