One reason why Friar Laurence's soliloquy in the beginning of Act 2, Scene 3 is important is that it helps to characterize Friar Laurence, especially by portraying him as a bit unusual. Friar Laurence is out at dawn picking herbs for potions. While it is not unusual for friars to practice healing and to know about the herbs and other natural remedies that are medicinal, it is unusual for a friar to be gathering herbs that promote both health and death. We learn that Friar Laurence is gathering herbs that both heal and kill in the lines, "I must up-fill this osier cage of ours / With baleful weeds and precious-juiced flowers" (II.iii.7-8). In these lines, an "osier cage" refers to a willow basket, or basket made of willow branches (Shakespeare-navigator). The term "baleful" can be translated to mean "poisonous(enotes)" and "precious-juiced flowers" can refer to flowers, or herbs, that contain medicinal juices. Since poison kills, we know that he is collecting herbs that can kill a person, or at least harm a person, as well as heal a person. If he were gathering herbs in a traditional sense, he might avoid the poisons and only take the herbs that heal. Hence, this passage portrays Friar Laurence as a very unusual friar and prepares the reader for the potion that he later gives Juliet as a solution to her problems.
This soliloquy also helps characterize Friar Laurence by showing us one of his philosophies, which explains many of his actions. While talking about both the virtues and vices that the earth creates through its "plants, herbs,[and] stones," Friar Laurence declares that virtues can easily become vices and that vice can be thought of as virtuous if there is justifiable reasons behind the vice. We see him say this in the lines:
Virtue itself turns vice, being misapplied,
And vice sometime's by action dignified. (21-22)
An example of a virtue turning into a vice would be Friar Laurence's good intentions in agreeing to marry Romeo and Juliet. He believed that the marriage would end the feud. However, he did it in such a secret manner that his virtuous intentions created disaster. Also, a vice, or action of Friar Laurence's that we can consider questionable, or immoral, is his decision to help Juliet fake her death. He felt that faking her death and uniting her with Romeo was in her best interest, so he decided to do it, even though he knew it was deceptive. Sadly, again, Friar Laurence's deception created greater problems than before.
Hence, this soliloquy is important because it serves to characterize Friar Laurence, showing us what he knows about herbs and also showing us his philosophy concerning virtue and vice.