Answer:
I assume you mean functions? A respectable tour operator, such as those I deal with, package segments of travel to achieve an end. This results in a higher value product at a cost less than the assembled segments.
For Example, if you wish to visit the Maya city of Ek Balam, you would first have to get to mexico, (round trip airfare), pay entry and departure taxes, and find your own transportation to a hotel you have booked for yourself and your baggage. You would then have to make yourself understood when you get two double beds instead of a king, and a "garden view" instead of an "Ocean view". That's when you learn what that "run of the house" room is. It's what they have leftover that they want to put you in. Then you have to find a tour guide to take you to and from Ek Balam. You pay entrance fees for yourself and the guide. You listen to the guise explaining the architecture, history, and customs of the "Mayans".......... and discover you know more than he does.
While enroute to/from you want to eat, maybe get a drink, but, what do you know. You are totally in this guides hands.
Our way, you drop your bags at the airport, pass them to a representative at the other end, and see them in your room later in the day. All fees and taxes are paid, no extra baggage (unless u r really bad) charges, no trying to figure out how many times you passed the same place enroute to a hotel you don't know. Each property is selected based upon performance. Happy clients make repeat buyers. As for guides, same thing. You'll get an INAH student, or someone who has worked excavating the site during the season, or someone equally qualified, who SPEAKS and UNDERSTAND idiomatic American English.
I could go on ALL day. Where ever you are going................. The Nile, The Amazon, The Danube, or the Mississippi, your BEST bet for enjoyment and VALUE lies in finding a respectable tour operator/packager.