silver suitcase's in trunk

Either the sequence where their purpose is shown to us has been cut or in those suitcases was hidden all the equipement needed for the museum theft - given the fact that Tess had to get herself disguised in the car, therefore becoming „Julia Roberts” and getting off at the hotel right before the big improvised theft.

Hope I managed to answer your question. (btw, I incline for the second explanation)

It's all part of their "Julia Roberts" disguise - you can see Turk bringing the suitcases into the hotel behind her. In-universe, they're there because a glamorous Hollywood movie star accompanied by an entourage would not turn up at a top hotel with two dirt-cheap black duffel bags for their luggage - or at the very least general public perception is that they wouldn't. I mean, if you worked at a 5 star hotel and [insert A-List celebrity here] turned up to check in with nothing but an old backpack, you'd think it was more than a little odd, especially if they had three other people in tow fawning over them.

Out-of-universe, it's just a continued part of the escalating set-up for the joke: several times now the audience has heard various characters claim that Tess looks like someone famous, we know that whatever plan Linus, Basher and Turk have come up with centres around Tess' resemblance to said famous person, the suitcases (along with the next scene of the group in the car dressing her up) are just another piece of set-up for the punchline (which comes when the manager at the hotel specifically name-drops Julia Roberts). At some point during the set-up, the audience is supposed to at least halfway figure out what the punchline is. The "classic Hollywood star" suitcases are just another element to try and help those who haven't yet worked it out.