What is the sense of the defend token? The rules say it is changed into a guard token immmediately.

The defend token still counts toward the order limit of 4. So if you leave the defend order until the end of the planing phase you can keep track of all the orders placed, at least that is what I would do.

The token itself has no use, really. I think they had some material over ^^ You announce to play it, place it and instantly discard it to set a guard token, so you can just skip the step of placing the guard token and simply announce placing the guard token. And yes, you can do that up to four times per planning phase if you wish

But you are restricted to 1 defence order per planning phase.

Rules:

These unique orders are never placed in an order stack,
and are instead placed faceup in an area and immediately
resolved during the Planning Phase. These orders cannot
be used for the Event card option (to draw an Event card
instead of executing it). Each player may only execute one
Defend order per planning phase. These tokens have a
unique shape and color to remind players that they function
differently from most orders.

Also in addition to giving a Guard token, it allows you to do a mobilize to the area on a planet that you placed the Defend order. So you pick one friendly area on any planet, and then can do a mobilize to that area, and you also get a Guard token there in the same area.

Why the **** - if they are restricted to one per planning phase, then why are there more than six guard tokens? Very *****.

Guard tokens can stay for more than 1 turn.

Ah, ok, that explains it ^^

But still, what's the point of place an order and then immediately change to the token? If it's on the computer then I guess it would make sense since the computer will do this "unnecessary" step for you.

It is fully unnecessary. They had paper left or something else. Perhaps for visualization. PErhaps that you have to time to rethink where to put the guard order in the time needed placing and discarding the order. Perhaps so you don't forget that you have placed 4 orders, though only 3 are on the board. No idea. You don't really need it, that's for sure.

There are other ways to get guard tokens as well. For instance Dugalle's stage 3 leadership card that allows you to place 4 guard tokens in areas you control.

So the act of placing a guard token doesn't nessessarliy give you the addition of a mobilize action as is the case with the Defend Order.