There are many electoral monarchists showing their vices and making communists blush with their talk of guillotines and beheading monarchs. This is a shameful state for a monarchist to be in, electoral or hereditary, but that won't be the topic of this thread. Rather than discuss people with a vendetta against monarchy (whether it is absolute or plain hereditary; mostly narrows to hereditary), I'll have to be an apologist for hereditary monarchy. This is because these people basically come down to a hatred of primogeniture succession. For the reason of centralizing land and favoring the eldest son unequally over the other sons.
>centralization
Monarchists who favor hereditary principle and yet still like primogeniture have to wrestle with two extremes. There is the aristocrat-lover, ultra-individualist who hates nations and nationalism in favor of micro-states and divided up counties/prince-electors, and then there is the nationalist who wants the big national state with democratic anti-primogeniture sentiments. This comes down to the age-old struggle between monarchy, aristocracy, and democracy. The monarchies of the WW1 period in Central Europe reached a middle ground between nationalism and royalty. There were two German states, but an encompassing pride for nation regardless. This might upset nationalists who favor complete unification while seeing multi-ethnic peoples together in hereditary empires. Hereditary empires unified people thanks to primogeniture (without electoral succession) thanks to gavelkind succession becoming less practical. Centralization organically grew as land became more indivisible because the eldest son inherited land and the land remained tied and incorporated more and more people on said land rather than constantly unifying and splitting it up again.
The electoral monarchies were strongly hereditary monarchies and those that weren't would later become hereditary with the rise of primogeniture. I don't think the electoral process of aristocrats would avoid centralization, despite primogeniture bringing people under a fold. Usually centralization and decentralization are a rising tide and happen regardless, especially when empires grow larger and larger; but this doesn't mean they have to be overbearing on liberty. An anon pointed out that the Russian Empire, despite centralizing with the Romanov dynasty, still was fairly better off in the later centuries despite abolishing serfdom.
>how about hereditary roots?
While electoral monarchists can boast about being older in a sense with Germanic states favoring electoral bodies, there are a few exceptions (especially outside of Europe). Some electoral processes among those were incredibly superficial and strongly hereditary like the Byzantine Empire (despite laws) and other cases. There are also those monarchies that didn't establish themselves by right of vote, but by conquest in certain niche cases with the monarch winning a battle and declaring himself king. As for other monarchies, this could involve the Kingdom of Wessex with its tales of kings and its strongly hereditary monarchy, the things surrounding the strongly hereditary realm of Norway, and so on with the Rugii Kingdom and other exceptions to the rule.
>the evolution of monarchies
In France, the Merovingian dynasty started electoral but later became strongly dynastic and hereditary. Those kings played roles such as being judges and ruling idly. Until the electoral office of the Mayor of Paris and the kingship merged together with Pepin the Short's takeover and the establishment of the Holy Roman Empire. After that, the House of Capet started the longest succession of primogeniture in France and the electoral process vanished in 1,223 AD.
Between 1,223 AD and 1,670 AD, most monarchies in Europe became hereditary. In most of England, the Norman Conquest (offically in early1200s) . In Scotland, primogeniture started to exert itself in 1,371, Portugal, and Sweden later in 1537. Outside of Europe, there were plenty of civilizations with strong dynasties like Zhou Dynasty in China and Egypt's dynastic rulers. Imperial Japan was also the longest hereditary dynasty to boast.
>how did it become necessary and how did kingdoms/nations develop?
Primogeniture became necessary with growing military conquests and the need for indivisible lands. The development of many nations owe gratitude to this process and these monarchies. I think nationalists should recognize this. While aristocrat electoral monarchists should also recognize that the electoral process even in decentralized states can evolve to become federal (like the United States in its development).