Airline change fees are big business. In the US, changing your flight can cost you $200 just in penalty fees and that is not the end of it; you also have to pay any fare "uplift". "Fare uplift" happens when the flight you wish to change to, only has more expensive seat prices than the one you want to change from.
Change fees on low fares is more-or-less understandable; If you have only paid $50 for a flight, then expecting the ticket to cost money to change is understandable (if, of course, changes are allowed at all) and if you don't travel, then not getting any money back, is also (arguably, I suppose) acceptable.
However there is hidden competition in terms of change and refund fees as you go up the fare scale, depending on airline, on many routes (not on all routes; mostly long and medium haul). Something a proper GDS equipped travel agent will assist you with, but a website will not. (A website will usually present the rule facts, if you know where to find them, but on balance, websites use the fare total as being the main choice criteria)
Lets take an example:
United Airlines will fly you from London LHR to Washington IAD on the 15th November, back on the 26th for £689.77. If you cancel on this fare you lose the lot - well not quite, because you get the taxes back, which comes to £164.77 (as long as you remember to claim them, that is). The change fee on this fare is £130 ($170-ish).
Then the picture changes. If I pay £839.77 for the same trip, I can cancel the whole trip and United will charge me £190.00 - changes will still cost £130, but for the extra money I have a tangible benefit - I can cancel and get back £649.77. If I pay £1,236.77 and cancel, I get all my money back and if I change at that £1236.77 fare level, there is no penalty charge.*
What about doing the same trip on another airline?
British Airways offers the same trip for a best price of £701.77. Again, if you cancel you lose the lot, save for .... £164.77. With British Airways, I would have to pay £1,581.77 before I have a fare which I can cancel, though even then I have to pay a £150 fee; changes at this fare also carry a £150 fee. If you want a fare where you can get all your money back and pay no penalty on any changes, then with British Airways, I would need to pay £2,149.77.*
Apart from "use a proper travel agent" the important message, here, is that it is not the airline you travel with or if that airline offers you a free seat reservation, bag or cuddly toy and certainly not the cheapness of any fare that should be the main decision-influencing factor - I have always said that the best airline in the world, is the airline that gets you where you need to be, when you need to be there.
The starting point for any decision with regard to a trip should be the reason for your trip and the factors that may influence any changes to that trip. Based on the correct influencing factors, then a decision can be made about the fare that best suits your purpose. The name on the side of any aircraft is, frankly, academic.
*Note: Fares and fare rules taken from actual bookings made 16 AUG 17 using the Sabre GDS.