> The choice to store two lengths in the packet header means that you can record both the actual wire length--the number of bytes that came in--and a bigger number that says where the next record starts, and any metadata you like in the gap.

> There is a very complicated "NG" pcap format, but nobody uses it.

This is the reason that pcapng was created - to formalize the structure of this metadata. pcapng "Pcap Next Generation" (2006) is an update to the pcap format (which originated with libpcap/tcpdump in the 90s). It's used in the networking industry as it's the default output file of wireshark, tshark, and tcpdump to capture traffic. If you use these tools, chances are that you have pcapng files.

The biggest reason to use it over pcap (in networking) is that you can save packets from multiple network interfaces. If you have a wireless access point and want to capture both ethernet and 802.11 traffic in the same capture, pcapng would be necessary.