2. Its effect on the Bracha Rishona and when one should recite the Bracha Achrona:

See Chapter 19 Halacha 5 for the full details of this subject!

Causing one to repeat the Bracha Rishona: The intentional recital of an after blessing after eating a food or beverage requires for the before blessing to be repeated if he were to decide to continue eating or drinking after its recital. However, in the event that one planned on continuing to eat and drink, and mistakenly [i.e. absent-mindedly, or due to a mistake in the law] recited the after blessing, then it is not necessary for him to repeat the before blessing prior to continuing eating and drinking.

Not to say the after blessing unless one is done eating: Due to the above reason, one may not recite an after blessing if he plans to continue eating or drinking afterwards in order not to unnecessarily cause himself to need to repeat the before blessing. However, if one will be taking a break and is concerned about losing the chance to say the after blessing, it is permitted to recite it before resuming. Likewise, in all cases that there is a Halachic doubt regarding whether a certain blessing must be repeated, then it is permitted for one to recite an after blessing in order to escape the doubt and force himself to be required to repeat the before blessing prior to continuing eating.