You can only have so much weight per (down) axle. Most 53 foot vans are 12k on the steer, 34k on drives 34k on the trailer. The axles have a weight rating. Ive never pulled flatbed but some of the loads I’ve seen aren’t evenly distributed on the trailer due to varying sizes. The lift axles make the trailer more versatile. You’ll know more once you know your weights. You don’t want to turn with them down though, the tires don’t really like that
Drop axles a for extended weight. A Quad axle trailer can go 54k on that bridge. It does vary in certain states. Lift them when your mty or under the extended weight on that group of axles. Extra weight, extra brakes. Roll easy. GVW 102,000 to 104,000. Depending on your truck.
Extra axles = more wt. to legal allowable. Generally 12,000 steer- 34,000 tandem ( spread of 8'6" will start allowing more wt. ). Spread of 9' or more is considered to be 2 single axles and allow 20,000 on each . Tri's are allowed 42,500 legally in most states on interstate ( bridge law) however, more axles you add to standard trailer is more empty wt . Regardless of number of axles your running ,you're only allowed 80,000 gross (unless permitted for more). You could use a extra axle to legal up and not need to move the load forward or aft such as , 12k steer- 26k drives- 42k tri. An extra axle is put up toward center of trailer to reduce strain on main beams of the trailer and allow more distribution of wt but , mostly used on straighter roads , tighter turns will increase strain and tire wear.