Some dark crystal and roast malts contain an appreciable amount of acid. If you use those in what I personally consider to be sensible quantities you shouldn't have low mash pH and it shouldn't be necessary to add chalk, bicarbonate or lime. But as home brewers are experimenters they sometimes want to use proportions of these in more than what I consider sensible quantities and in those cases mash pH can be low and alkali must be added to the brewing water if it isn't already sufficiently alkaline.