So I always wondered why it was so important to waste human life by throwing them against the walls of a castle when invading an area? It seems, in almost every situation I have read about or seen (with the knowledge that most of it is fiction), that its a better strategy to have a token force surround a castle, to keep an army from using it as a staging point, and to keep the people in, and then use the rest of your forces to just settle the land (now that everyone who used to live there probably all escaped to the castle).
I'll answer this as quickly as possible. The idea that most sieges ended up with a 'storming' attack, what you refer to as throwing men at the walls and hoping to break in, is a woefully inaccurate portrayal of Medieval European siege warfare. It is primarily borne out of fiction, as you hint at. Realistically, few armies or commanders were willing to launch the bulk of their forces against a well made and manned wall or keep.
You ask why didn't they just bottle up the enemy fortification and occupy the surrounding area. What you've described is essentially what they actually did. Sieges were protracted affairs where the besieger would try to prevent the besieged from escaping or receiving reinforcements/supplies, hoping to wear them down by starvation or thirst, or alternately use siege weapons/tactics such as trebuchets or mines to render the enemy fortification indefensible.
You bring up this long-term siege like it would be a 'perfect' tactic. Far from it. The besieged often had allies outside the castle - many battles throughout History involve a relieving army battling the besieging army to save the garrison in the castle.
Spreading your army out to harass the enemy countryside while ignoring the enemy fortresses is actually probably the crux of medieval warfare. It doesn't get shown in fiction much perhaps because having a protagonist riding around burning villages down and killing/raping the populace isn't too popular. Showing a protagonist heroically storm the walls of the enemy castle? Far more exciting.
Essentially your question paints an inaccurate picture, but your suggestion of 'why didn't they ignore the castle' provides the correct answer.
I will provide more sources when I get off my phone. Hope this helps
use the rest of your forces to just settle the land
/u/cazador5 has already answered the broad question effectively, and I'm sure other experts could add more. I just wanted to pick up on this one point.
If we're talking about warfare in Medieval Europe, generally the aim was not to kick off the original inhabitants and "settle" the land. All those peasants, merchants, clergymen, etc. living in an area were valuable. Many of them wouldn't particularly care if their taxes went to a French or an English noble. So as a leader you'd probably want to make sure you had loyal people in key positions but you wouldn't be replacing the inhabitants wholesale.
War goals could vary a lot, but on the whole your aim would be to bring your opponent to terms, by causing them harm - eg. to their finances, reputation, control and personal feelings. You might attack a ruler's subjects to prove they are incapable of doing their job and protecting their realm. (And losing castles was a good sign a ruler was losing control.) You might capture (or kill) relatives, friends and supporters to force your opponent to the table.
Exactly what you were trying to achieve would affect your attitude towards castles, whether they needed to be besieged, if they had to be captured, and if so, how urgently.