To be optimistic means to believe that there will likely be a positive outcome to an unknown situation, or to have a positive outlook, generally believing the best about people and things.
Ex: Though many at the office are worried that the new construction will not be finished by opening day, I am optimistic that it will.
Ex: Whether or not you invest your money in a second home has much to do with whether you feel optimistic about the economic future.
To be pessimistic is to believe that there will likely be a negative outcome to an unknown variable, or to have a generally negative outlook on people and situations.
Ex: Pessimistic is the opposite of optimistic. At the end of his difficult life, Mark Twain grew very pessimistic. His writing, although retaining his signature humour, takes a decidedly darker tone in his later years.
Ex: I find that I am constantly having to ask my cousin to refrain from her pessimistic statements about family members when we go to visit her. She is becoming a very unhappy woman in her frustrations about money and work.
Beneficial means that something contributes to a goal, task, or person in a positive way. In order for something to be beneficial, it must be good for the person, task, or goal.
Ex: Even though the words were difficult to hear, I believe my final project was better for the changes made after hearing some beneficial criticism from my colleagues.
Ex: It is hard to believe that anything that tastes as bad as that medicine could be beneficial to the body, but I suppose I can trust my doctor anyway.
A fringe benefit is something that accompanies one's salary or wages, such as insurance or vacation time. The term is also used an an expression to mean something that is an extra advantage to something you already have, such as the benefits of being married to an electrician when you need wiring fixed.
Ex: A particular fringe benefit for my sister Susie, a school maths teacher, is having her summers off.
Ex: Being married to the university's head custodian gives me the fringe benefit of always getting back into my office if I forget my keys