Why hold'claim Osgiliath?

[Gordis]

What seems curious to me is that the last Bridge has been built not too long before the War of the Ring:

For that is the ruin of Osgiliath on either side of Anduin, which our enemies took and burned long ago. Yet we won it back in the days of the youth of Denethor: not to dwell in, but to hold as an outpost, and to rebuild the bridge for the passage of our arms. And then came the Fell Riders out of Minas Morgul.- Minas Tirith, LOTR

Denethor was born in 2930. Sauron returned to Mordor in 2942 and declared himself openly in 2951, when Denethor was 20. What a crazy idea it was to rebuild the bridge at this time! Makes me think of Turin and Nargothrond...
Cast the stones of your pride into the loud river...

But the other side made strange moves as well. What was the need for Sauron to attack the Bridges of Osgiliath a year before the war, in June 3018? In UT it is explained like this:

So it was that Sauron prepared two strokes – in which many saw the beginnings of the War of the Ring. They were made together. The Orcs assailed the realm of Thranduil, with orders to recapture Gollum; and the Lord of Morgul was sent forth openly to battle against Gondor. These things were done towards the end of June 3018. Thus Sauron tested the strength and preparedness of Denethor, and found them more than he had hoped. But that troubled him little, since be had used little force in the assault, and his chief purpose was that the coming forth of the Nazgûl should appear only as part of his policy of war against Gondor.

Well, the nazgul may have feared water and all, but I don't buy it that there was no other way to ship them over the Anduin than using the Bridges. After all, Khamul and his pal from Dol Guldur did use a boat and were none the worse for it...
And the disadvantages were enormous. Basically Sauron gave ample warning to the Gondorians hitting a year in advance exactly in the same place where he planned his main attack for the next year. He sacrificed the Bridges, which he otherwise could have used next year. I guess the WK had a hell of a time shipping Grond and other heavy equipment over the Anduin in March 3019.