I think the point is what question are you trying to answer.
if all you ask is who won the most tournaments (slams, Tier 1) then all you need is the headline number.
if you ask who is best at slams though you need to look closer at the distribution. A hypothetical player with 30 Wimbledons would have the most slams but would not be considered the best at slams, just the best at Wimbledon.
in the Fedal comparison if Nadal beats Federer in overall titles but Federer beats Nadal in 3 of the 4 slams that‘s a relevant point.
One question is whether, when thinking about distribution, we should compare Slam to Slam (e.g. Federer at Australian Open v Nadal at Australian Open) or by ranking the Slams in order of preference (e.g. Federer at his second-favorite Slam v Nadal at his second-favorite Slam). Here's how it works out right now by the two methods:
A: By particular Slam
1. Australian Open: Federer 6-1 Nadal
2. Roland Garros: Nadal 13-1 Federer
3. Wimbledon: Federer 8-2 Nadal
4. US Open: Federer: 5-4 Nadal
B: By Slam ranking
1. At favorite Slam: Nadal 13-8 Federer
2. At second-favorite Slam: Federer 6-4 Nadal
3. At third-favorite Slam: Federer 5-2 Nadal
4. At least-favorite Slam: Federer 1-1 Nadal. Tiebreak: finals reached. Federer 5-5 Nadal. Second tiebreak: semi-finals reached. Federer 3-1 Nadal.
So, right now, Federer leads in three of the four Slams by either metric. But if you change to the second one, things get more complicated. It becomes harder for Nadal to catch Federer at their second-favorite Slams, as he needs two more US Open titles to tie, and not just one. However, it becomes much easier for him to catch Federer at their weakest Slams, as he overtakes him with just one more appearance in the Australian Open final. If Nadal were better than Federer at both their favorite and their least-favorite respective Slams, the case for Federer's superior distribution might hold less water.