Waiting can be appropriate when your child is meeting most milestones on time and the concern is isolated to one area. For example, a 15-month-old who is not yet walking but is cruising, pulling to stand, and meeting all other motor milestones is likely fine. Similarly, a toddler who understands language well but is not producing many words yet may simply be a "late talker" — studies suggest about 50-70% of late talkers catch up by age 3 without intervention (Rescorla, 2011).
The "wait and see" approach is less appropriate when you observe multiple areas of concern, regression (loss of previously acquired skills), or when your child is not meeting milestones by the outer edge of the expected range. Understanding the difference between developmental milestones and growth milestones can help you pinpoint which domains need attention. In these cases, evaluation is warranted regardless of whether your pediatrician suggests waiting.