Dynamic Linking Libraries in C++

Question

I was following the liveoverflow course about binary exploitation, when i got trouble following a part of his video. In the video is shown a binary linked to a library which calls the function puts, on the video is shown that before executing for the first time the function the GOT was empty and there was no way for the binary to know where the function lived, when i compiled my ELF binary i found out that before the _start function is even called those addreses were filled with the address of puts. I really can't understand why such thing happens.