Hi Lisa! The precepts are primarily trainings in avoiding negativities and thus keep people from experiencing the karmic consequences of engaging in killing, stealing, lying, sexual misconduct and intoxication***, People also accumulate merit as a result of avoiding killing, stealing, lying, sexual misconduct and intoxication.
My lamas have taught that it is permitted to take 1-4 vows rather than all 5 lay vows (although I don't know of a lama actually giving fewer than the 5 lay vows). This is also detailed in "Treasury of Precious Qualities"*; the outline follows: The Precepts of Bodhicitta and Action (Chapter 9) -> The Paramita of Discipline -> The Discipline of Avoiding Negatice Actions -> The levels of ordination -> The precepts of laypeople (pg 197).
Also included in that section is a discussion of the practice of taking the eight Mahayana precepts for 24 hours (which are primarily for laypeople who want to observe the major precepts of a monastic for a day at a time without actually having to ordain permanently. Also included in that section is a statement that people who do follow the eight Mahayana precepts for life are laypeople "after the manner of Chandragomin."
In the section "Avoiding negativity according to the Mahayana", the section immediately preceding "The precepts of laypeople", there seems to be the implication that following the precepts trains the mind in renunciation and faith (this is less of an implication and is basically stated directly). All of this however is situated in a discussion of the behavior of a beginning Bodhisattva (thus emphasizing the Mahayana view):
Avoiding negativity according to the Mahayana
Thus, for the beginner on the Bodhisattva path, the first task is to discipline the mind. The most important thing is to train oneself in such a way that the qualities of renunciation and faith constantly increase. Beginners must train to bring their minds under control, progressing steadily day by day and month by month. Indeed, if at the outset, when the mind has not yet been purified by the discipline of avoiding negative behavior, too much emphasis is placed on the other two aspects of bodhisattva discipline (namely undertaking positive actions and bringing benefit to others), many hindrances will occur that impede progress in the three trainings**, the path to buddhahood.
* a presentation of the entire Buddhist path from the Nyingma school written by Jigme Lingpa with commentary by Kangyur Rinpoche.
** sila (morality/ethical training, Tib: tsultrim), samadhi (concentration), prajna (wisdom, Tib: sherab)
*** the karmic consequences of killing, stealing, lying, sexual misconduct and intoxication are outlined in The Law of Karma (Chapter 3) -> Negative Actions -> The results of the ten negative actions and focus on negative consequences in future lives (which is interesting because some other texts emphasize negative consequences in terms primarily of rebirth in lower realms).
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche