Wire terminal Insulator questions

Hey guys, it's me again.

Most of my cables in my '74 Valiant have large cable insulation at the connectors / terminals. I'm not quite sure if it's actually insulation, as well as some sort of fusible link? I was just wondering what it's actually for and why it's so thick (I'm new to this electrical stuff and wanna learn more about it). What do I have to consider if I have to replace such a terminal? I'm thinking about "restoring" some sections in my wire harness.
I'm sorry if this was already asked earlier, if so just hit me up :)

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