1. When would you use Receive channel = omni? I imagine Voice mode being controlled by a keyboard controller playing one Part/one channel

When troubleshooting a system where you don't know or cannot determine the transmitting channel of the controller. There is not much call for having the tone generator set to Receive Channel = "omni". However, I can think of one use case, if you are using the Rack XS as if it were Performance mode, where four Parts are triggered simultaneously. It can involve playing the Rack XS one a single channel (not as a multi-timbral, multiple MIDI Channel device).

The most common use of the module would be to set the Motif Rack XS to receive global messages on a single receive channel, and set the unit to Multi Mode - where the MIDI Channel is determined on a per Part basis, and the Rack XS is 16-Part multi-timbral.

The global receive channel would be used to send a message to change the MULTI program, 1-128. But in Multi Mode, each Part has its own ability to receive a separate Program Change. By using MSB/LSB Bank Select along with Program Change messages, you are able to address the entire Rack XS, and address each of the 16 Parts individually.

2. Is there a a point of using program changes in Voice mode?
Or...if you do use program changes in Voice mode why not use Multi instead?

The Rack is built to be configured into many different types of setups. There is no one way to use it.

It contains the Motif series Library of Voices. You can configure these Voices into either four-way single channel configurations, or into 16 separate Parts on 16 separate channels for use with sequencers. You can even setup six Parts in mono mode for work with MIDI guitar synths.

So you can play it in Voice mode, you can create 4Part Layers, you can setup 16 Parts for sequence work... Your choice.

The allocation of effects is different in the different modes. There are 3 different Effect blocks. Each Voice has its own two Insertion Effects.
The Rack XS's internal mixer has a Reverb and a time delay (Chorus) Effect - these are considered "System" Effects, every instrument channel on this internal mixer has Send amount to the System Effects. Finally, everything goes through the Master Effects.