Early Modern European armies were by and large composed of professional soldiers who were difficult to replace owing to the time and cost of training and maintaining them long enough to become effective. Battle would risk the loss of many soldiers, and thus immense costs and the possibility of creating a window of vulnerability that other powers could exploit. Thus commanders might be reluctant to offer pitched battle for fear of irreplaceable losses.
If one army had chosen terrain or manipulated the circumstances such that they would be advantaged in pitched battle, their opponents would usually understand that they were at a disadvantage and could use a variety of techniques to decline any offers of battle when they were in a bad position.
Yet there were still battles in Early Modern Europe. Why did armies ever accept their opponents' offers of battle?
Battle offered the best, and usually the only, chance to destroy the enemy army, that is, to put in such a condition it can no longer fight.
Time is not often neutral in war: it is either on your side or the enemy's. In Latin, there is the proverb, beati sunt possidentes, 'blessed is he who possesses.' Most European economies at this time were subsistence based, with land being the greatest source of wealth; wars were being fought largely over territory intended to serve as a source of revenue, so actually taking possession of the disputed territory was a crucial element in strategy. This usually revolved around fortresses and sieges. In early modern armies, losses due to illness, exhaustion, and desertion were more or less constant, so the state's ability to make war was not infinite. Similarly, on a smaller scale, a fortress besieged can only hold out so long. As such, at any given moment, one side or the other was in a position to 'win by default' after a fashion; if things keep going how they're going, one will exhaust themselves first, leaving the other victorious.
The crucial strategic question then is judging whether time is on your side or the enemy's. When a general sees that time is against him, the onus is on him to change the strategic situation. When time is already on your side, making that advantage permanent (or sufficient for the foreseeable future) is also an attractive proposition.
Few things can rival the ability of a major battle to alter the strategic landscape. Before the battle, it's unknown which of the two armies is stronger -in terms of numbers, leadership, morale, etc.- than the other, so to some extent they can hold each other in check by mutual risk-aversion. A victorious battle, though, changes this completely. The very act of ceding the field is an admission by the loser that they were not strong enough to hold it; when one's best efforts have proven to be insufficient, there can be no expectation of success by fighting again under worse conditions. Wherever the enemy goes, you can go, and they must leave or risk total destruction. The use of victory means exploiting this inability to fight to the utmost.
Let's look at the wars of Frederick the Great for some examples. He conquered Silesia and its fortresses in 1740, though the fortress of Neisse remained under siege. In 1741, the Austrians invaded the province with the intention of relieving Neisse and taking back the places that had fallen to the Prussians. Turning the Prussians right flank left them cut off from their line of retreat. Assembling the army out of winter quarters, Frederick decided he had to put a stop to this. Both armies marched on Neisse, and the Austrians managed to relieve the place; if Frederick wanted to take it, now he had to defeat the army protecting it, both to get at it and to do so with secure supply lines. For the Austrians, leaving their position in the face of the Prussians would mean giving up their advantage. Frederick's victory in the ensuing battle of Mollwitz allowed him to seize the fortified city of Breslau, as well as conclude an alliance with the French. This gave him a stronger hand negotiating with the Austrians, who ceded to him Lower Silesia and allowed him to quarter his army in Upper Silesia; Neisse fell after a token siege.
That was Frederick's first battle. His last battle was Burkersdorf in 1762. In 1761, the Austrians had seized the key fortress complex of Schweidnitz in Silesia; the war was winding down, and whoever held the most fortresses at the end would be in the best position to negotiate their preferred settlement at the peace conference. The Austrian main army was encamped in a fortified position near Schweidnitz, and so the place could not be taken until it was forced to retreat. Frederick could not allow the fortress to stay in Austrian hands, and the Austrians could not simply give it up. Frederick furthermore needed to expedite his attack; part of his army was a Russian contingent which was recalled after Catherine II overthrew her husband and ended the alliance. Frederick had to make the most of them while they were still in position; they did no fighting at Burkersdorf, but they held the Austrians' attention to the front while separate Prussian columns attacked their flanks.
Not every battle in the Early Modern period revolved around the demands of positional warfare, but it almost always exercised a powerful influence over the decisions of the commanders.
