As someone who self-hosted email servers, here's why it is a big deal: we all know that HTTPS is basically HTTP with implicit TLS, which is a common knowledge and industry standard of today.

But back in time when people still don't realize the importance of encryption, and most computers process crypto primitives in software, to the time when SIMD was not even a big thing (3DNow, MMX, SSE...) and only some high-tech niches and games such as Quake and Unreal Tournament demands it (where we obviously know today SIMD is super important for everything that can be processed in automatic vectorization with advancement in compiler optimization tech, including the fancy matrix/tensor math you run with your transformers and LLMs), which means they are slow because back in 30 years people don't have the luxury to enjoy widespread cryptography, only few nerds are adventurous enough to go deep into the abyss, and worse, still have to fight a war with the US government that they have military exports control on cryptography [1], since that's when RSA (the company, not just the algorithm) was prevailing, and everyone is still discovering new applications of cryptography to the commercial internet, one of which was smart cards, but that's not convincing enough to the US government cryptography should be set free from controls.

Today we even have accelerators for AES called AES-NI and the same for SHA too, and we even have those on embedded systems, MCUs such as Cortex-M or ESP32.

And so, cryptography is not only slow, not readily available and researched throughout, but also having legal troubles 30 years ago, so the predecessors used something called "opportunistic encryption", aka STARTTLS in your email client. All it means is that you start a TLS session on demand with a plaintext command (heck everything before STARTTLS is plaintext) indicating the following data would be new TLS handshake and starts the state machine exchange process.

Basically, they want to make sure that if the outcome of crypto wars turned out to be favoring the government, ala the three letter boys (FBI, CIA, NSA), we can still have some countries that could use TLS, hence the opportunity part, but this is obviously hell because everything before STARTTLS including the STARTTLS command itself is subjectable to MITM.

Today this is a thing of the past, including the crypto wars which it eventually sided to the people and flourished all e-commence businesses because now we all can enjoy safe and secure internet, and we all agreed that implicit TLS is the only way to go. I would argue that the success of TLS is part of the reasons internet exploded to an exponential scale because now banks can make secure transactions without someone sniffing it, and increased liquidity to the internet businesses, and that itself is a virtuous cycle.

And some people dare to have the audacity and have attempted to even remove implicit TLS from official IANA port list in the form of RFCs, only to be met with protest. This is why there are a lot of misconceptions online saying that implicit TLS is bad (those RFCs to remove implicit TLS were initially accepted but then quickly retracted), and often give confusing information which port people need to use. Basically, they are reinforcing the reminiscent knowledge 30 years ago.

Protect cryptography and thus your privacy and security at all costs.

[1]: https://en.wikipedia.org/wiki/Crypto_Wars