How does a full node benefit from receiving a block with a version number in its header?
The block version number has traditionally been the way for miners to signal their readiness to enforce a soft fork. The version number was likely used because of its historical use for soft fork signalling. Also, the version number does not indicate that a particular set of rules are active for a block. In fact, the version bit used by Segwit is no longer set by miners (besides those using ASICBOOST) since Segwit has activated.

Additionally, the version number helps SPV wallets know when segwit is active so that they can use it. Otherwise SPV wallets would have to download entire blocks in order to examine the Coinbase transaction in order to do so, which would largely defeat their primary benefit.

Full nodes don't really benefit from the version number being used as the signal. However when deploying any kind of fork, consideration must be made for everyone in the ecosystem. Using the version number does not hurt full nodes, has some extra benefits of not taking up block space for transactions themselves, and lest SPV wallets know when the fork activates.


  I thought Segwit was about transaction ordering in the actual block


No. Segwit does not effect transaction ordering at all. It changes the format of transactions and how certain output scripts are validated.


  Other than protecting those values for future forks, does Segwit require any special consideration by SPV wallets / miners?


Yes. SPV wallets need to know when Segwit activated so that they know when they can make and accept transactions containing Segwit inputs and outputs.