A UAE offer letter almost always quotes one headline number, your total monthly or annual package, that bundles basic salary with a housing allowance and usually a transport allowance too. Because every part of that package is tax-free, the split between the three doesn't change what lands in your account each month at all. Change the split and your monthly take-home stays exactly the same.
Gratuity is where the split stops being cosmetic. Article 51 of Federal Decree-Law No. 33 of 2021 defines gratuity purely in terms of basic salary, 21 days' basic pay per year for the first 5 years, 30 days' basic pay per year after that, capped at 2 years' basic salary. Housing and transport allowances are excluded entirely, no matter how large they are relative to your basic pay. Two employees earning an identical total package can walk away with very different gratuity payouts purely because one employer structured 85% as basic and the other structured only 60%.
Since raising the basic-salary percentage on an unchanged total package doesn't cost the employer anything extra month to month, it's one of the lower-friction things to negotiate if you expect to stay several years, it directly increases your eventual gratuity at zero cost to either side today.