I know nothing about the latter tradition, but with the first communion - it really is just about showing everyone how much you pretend to be able to afford and begging lots of money from guests. I really, really cannot imagine why you'd invite 50 people to a first communion dinner. It's just... unbelievable.
Why is everything so black and White with you?
Don't tar everyone with the same brush as we had over 50 people to our daughters communion and it had nothing to do with showing off... It was more that the family wanted to get together for a party to celebrate her communion, and there was no alcohol at her request in keeping with the family's tradition on these parties of which was totally respected as it was her day.
No 'begging' was done (nice choice of sarcastic words) and our daughter was given a combined gift where the family paid for a bike and trampoline for her.
But that was not the main reward for my daughter the reward was that she was very proud of her day and she wanted to share it with her family, and when you have children I hope you will see the pride in their eyes as well.