Boats and especially Trawlers do not need trim tabs. All real boats will float and move through the water just fine without them.
What properly sized and installed trim tabs will do is enhance performance and provide a flexibility that is not possible without trim tabs. Boats are designed to run at a specific running angle and on an even keel. The dynamic environment boats operate in involve varying wind and wave conditions. The boats themselves have ever changing load conditions as people move about, fuel comes aboard and is burned, tenders load on davits, ice melts, and very large fish come aboard. Trim tabs can allow a vessel to maintain an even keel and efficient running angle in spite of all these variables.
Tab sizing is important. The faster you go a smaller tab is needed to be effective. The reverse is also true, the slower you go the larger the tab is necessary.
Not all boats will benefit from trim tabs. Some boats have so much weight aft that no size tab will help. Some like boats with canoe sterns will be very difficult to fit tabs, some boats don't have enough useable transom to accommodate an appropriate size tab, some are too slow and too small for tabs to work.
A great many boats, even many full displacement trawlers can benefit from trim tabs. On my own Down East hulled Mainship 34 w single 200HP I could only plane with light fuel, clean bottom and downhill with a tail wind. Install 12 x 42 tabs and she doesn't leap onto plane, but lumbers up just fine to 15 knots under most load and normal bottom conditions. At 8 knots a little tab helps he find her "sweet spot" with just a little bow up attitude to keep the forefoot from "wandering."
Trim Tabs are not for everybody, or every boat, but they do what they are designed to do for a great many boats and are one of the few accessories you can buy for your boat that can pay for themselves with fuel savings.