The DNS protocol can be layered over UDP or over TCP. In its original form DNS/UDP has some quite draconian packet size limits that are reached quite quickly in the modern world. Originally, this mandated falling back to DNS/TCP. But TCP is significantly more expensive as a transport protocol, especially as the client has already had to try to perform the transaction once over DNS/UDP before falling back to it, and trickier for servers to implement than DNS/UDP.
EDNS0 ameliorated this greatly, allowing clients and servers to keep talking DNS/UDP without falling back to DNS/TCP, up to much larger packet sizes. That is primarily why one would want it, even if one did not want any of the other things that it incorporates.
While packet size is one thing you can do with EDNS, it’s really a mechanism to allow the DNS protocols to add new features, as there’s no version in the DNS header.
Also, a lot of DNS hosts do not allow for large packet sizes over UDP to attempt to reduce the effect of reflection attacks.
"The use of the EDNS(0) padding only provides a benefit when DNS
packets are not transported in cleartext. Further, it is possible
that EDNS(0) padding may make DNS amplification attacks easier.
Therefore, implementations MUST NOT use this option if the DNS
transport is not encrypted."
Apparently it does have a padding proposal, but it wasn't thought through very well. They only had the use case of confidentiality in mind, and decided to deal with amplification by forbidding cleartext use, no matter what the response:request size ratio is.
basically small clarifications and enhancements of the original DNS standard with rather huge impact which allow the functioning of modern DNS features like DNSSEC (due to the need for larger messages, which need EDNS compliance, as EDNS mandates larger minimum supported message sizes), and EDNS in itself is a signaling mechanism to indicate support for these newer features (like bigger message sizes, DNSSEC, etc)
If I remember correctly, the big thing about EDNS is that DNS UDP datagrams were originally restricted in size to a total length that would be unlikely to see IP fragmentation on a network of 1980s hardware, meaning that in practice DNS packets had to be much smaller than the maximum reasonable size of an IP packet. In addition to setting up extended options, EDNS0 also allows DNS packets to be large.
EDNS0 ameliorated this greatly, allowing clients and servers to keep talking DNS/UDP without falling back to DNS/TCP, up to much larger packet sizes. That is primarily why one would want it, even if one did not want any of the other things that it incorporates.