Most likely because linear probes are only good for about 4 cm of depth, due to the high frequency relatively low penetration. Past 4 cm, anatomy becomes cloudy and difficult to identify. Most likely if you are doing an LP on an obese patient, the depth can vary form 6 cm to 13 cm, in some cases. If you have a skinny patient, linear probe would be fine, but most of the time you can actually see the spinous process and your landmarks, and wouldn't need an ultrasound. Hope that helps.

Direct the needle cranially rather perpendicular to the skin

probably didn't change much with ultimate outcome - but it really helped the team know when there was contractility and good compressions - and when people were getting tired. also to know the severity of the situation with the air in the hepatic vasculature and help people when it was called at the end the more you look however, the more you learn and so mostly this was learning how to time POCUS in a resuscitation and use it to guide good compressions - this is probably the most important