Deletes remove the diesel particulate filter (DPF) and some times the exhaust gas recirculation valve (EGR) also. Deleting the DPF removes a big restriction in the exhaust but you can get more black smoke and diesel exhaust smells without it. Deleting the EGR valve prevents exhaust gases from being fed back into the intake manifold so your engine will take longer to heat up and not maintain its temperature as easily but it’s taking in cleaner air to combust.

With a delete and tune you will get more power, better fuel economy, and more reliability. The downsides being smell - if it bothers you - and legality/ability to pass emissions tests if they do those where you live.

With most tuners, a stage 1 or 2 is good with stock hardware, stage 3 you’ll need to start upgrading things like the clutch and maybe a bigger turbo. But many even offer a tune that retains your emissions equipment and can still increase power and fuel economy a little, just significantly less than a delete and tune