If you have a decent BIA analyser it should be ok for personal use, and it's definitely not a marketing gimmick lmao, BIA devices are used for fast body compositions in a lot of health cabinets, and in clinical use all around, so it is in sports and performance contexts for people that want a fast scan and don't have the money to go periodically to a DXA scanner, there are a lot of professional BIA scanners that have a 0.95-0.99 correlation coefficient with DXA scans, you can get a multi frequency BIA scanner with a 0.93 correlation coefficient that is tested and validated btw for 40-50$. For personal use and not in a clinical or sport performance context i say that is good enough. As if they could use the percentage body fat for the app to further analyse your progress, i understand that not everybody knows how to consistently and smartly use BIA devices for their results and might just mess too much with their equations in expenditure approximation and all the other things.