They don’t need to know if you plan on buying a car today, this week or next week. People are more vulnerable under high-pressure when alone so having others there with you can reduce the chances you rush into a decision. If you are using the phone to call around with questions during your preliminary shopping it is the salesman’s job to get you into the showroom.
If they sense urgency they will immediately realize they have some additional bargaining power. This can also help to ensure the salesperson is completely forthright. They may lure you in with specials or deals that may or may not exist so it is up to you to do your due diligence beforehand and strictly get the facts you are looking for.
The salesperson will certainly ask if you will be trading in but you don’t have to tell them yes or no, maybe you are considering it.
Either way, if they know you will be trading in they will use this to their advantage in the negotiation which will undoubtedly become more confusing and potentially cost you some money in the process.
Once you are in the showroom it is their job to send you home with a vehicle.