The 24 word mnemonic is the encoded (not encrypted) form of a very large random number (i.e. 2048^24). Your root private key is derived from that random seed. All other private keys in the BIP44 hierarchy are derived from the root key. Public keys are derived from their respective private key.
In short, your 24 word mnemonic is the root of everything and can therefore be used to restore the key hierarchy on another device that adheres to the same standard.
A HW wallet comes with a guarantee that the root key (or any other priv key for that matter) will never leave the bounds of a special security chip on the HW wallet. Additionally, it adds another layer of security, which is related to the F2A principal: To gain access you must KNOW something AND must HAVE something.
If you give you 24 words to a piece of software, the attack surface is massive. In case your computer gets lost/stolen the attacker has all the time in the world to extract the root seed from it.
There is really no ground to stand when arguing the benefits of a SW wallet - definitely not from a security perspective and not even from a convenience perspective. I can travel with my HW in the pocket to any internet terminal and have instant access. The device can get lost/stolen/brocken any time - when an attacker gets the unlock PIN wrong three times - it erases itself.