As I understand it, Calcium in the mash has at least this list of benefits (to which there may be additional benefits):
1) Stabilizes the Alpha Amylase enzyme so it does not get degraded as rapidly at mash temperature.
2) Precipitates malt oxalates in the mash that may lead to downstream haze issues and potentially even bottle gushers (if not precipitated in the mash).
3) Liberates H+ from malt phosphates and thereby lowers mash pH.
One big difference that I see between US and UK home brewing is that in the US we tend to presume that around 50 mg/L of Ca++ ions is generally sufficient in the mash, whereas in the UK the perception (or rather, my perception) is that it may be more like 100-150 mg/L as the desired range for Ca++ ions. That plus we don't have CRS/AMS here, and this is perhaps because (IPA's excluded) we don't typically target as much chloride or sulfate in our beers. And lastly in the USA we generally advise to not add magnesium, or at least to keep it on the very low ppm side, with the perception that the only thing it can contribute is an undesirably nasty/bitter flavor. I doubt if there are any hard and fast rights or wrongs here. Our differences in approach may stem from a more German influence to US brewing. ???