Selecting a favourable moment to begin something is among the oldest applications of astrological technique. It works differently from interpreting a birth chart and requires a different apparatus.
The question is forward-looking
Interpreting a chart for a person begins from a fixed moment already past, whereas choosing a time begins from a range of future moments and selects among them.
The practitioner is therefore searching rather than reading, examining many candidate times against criteria for the specific undertaking in question.
That search is laborious without tables or computation, which is one reason the technique has always been associated with technically trained practitioners.
Criteria depend on the undertaking
Different activities are assigned different requirements in these traditions, so what is considered suitable for a journey is not what is considered suitable for a marriage or a foundation.
Handbooks in several traditions list categories of activity with conditions attached to each, forming an extensive and highly specific body of instruction.
The specificity means the practice cannot be summarised in general rules, and practitioners work from detailed material rather than from broad principles.
Practical constraints limit the choice
Available times are constrained by everything else: the availability of participants, the season, the harvest, travel and the requirements of other institutions.
Practitioners therefore work within a narrow window and select the best moment available rather than the ideal one described in the handbooks.
The negotiation between the technical criteria and practical necessity is a routine part of the work and is discussed openly in the literature.
It shapes calendars and public life
In societies where the practice is established, it affects when weddings occur, when businesses open and when construction begins, producing visible seasonal clustering.
Printed almanacs listing suitable and unsuitable days circulate widely in several traditions and are consulted by people who otherwise engage little with the subject.
This gives the practice a social footprint considerably larger than the number of people who would describe themselves as committed to it.
Why the technique persisted
Choosing a time provides a structured way of deciding among options in situations where the considerations are numerous and no decisive argument is available.
It also fixes a date, which allows preparation to proceed, and a settled date has practical value in coordinating many people around a single event.
These functions operate whatever a participant holds about the underlying framework, which is part of why the practice survives in settings where the framework is disputed.