You need historical context for this.
To the north of Laodicea, Hierapolis was located on therapeutic hot springs.
To the south, Colossae had cold springs of clean, drinkable water.
Laodicea had problems with its water supply, which was brought in by aqueduct
from a few miles away. By the time the water reached Laodicea,
it had become lukewarm. It was at times tepid, unclean, and undrinkable.
It could cause illness, so that you might want to spit or vomit it out,
as Jesus is says he might do, metaphorically, with the entire Laodicean church.
Laodicea seems to have absorbed the lifestyle of the wealthy and elite of the city.
They had a kind of social sickness rendering them neither hot nor cold.
Not good for anything, in a spiritual sense.