There is no good reason not to have a letter of recommendation from your current boss. While it may be awkward to ask for in some cases, a letter from your boss bears a lot of weight. Your boss can attest to your current strengths and weaknesses the best. The only reason not to have one is if your boss wouldn’t recommend you to anyone.
If you are contemplating graduate school or other educational opportunities, you likely will need a few letters of recommendation. Even if you aren't sure when you will go to school, it is good to have a letter from your boss on hand. Graduate school deadlines creep up quick, and it can be difficult to get a letter from your boss on short notice. It is easier to be prepared and ask for a letter before you actually need one. This way your boss has plenty of time to write it -- and you will have one ready, whenever you need it.
If you think that pink sheets loom, it might be time to prepare for a layoff. Although it could be premature, it is better to be safe than sorry. Having a letter of recommendation from your current boss, written before the ship sinks, might help you land a new job. Your current boss' recommendation will be highly valuable to new employers, as your current boss ought to know you best.
Recognizing Your Accomplishments
If you have had recent success at work, it makes sense to get a letter of recommendation out of the deal. This puts in writing the accomplishments you have achieved. You never know when your boss might take another job or otherwise leave the company. It makes sense to ask her to write a letter after an accomplishment, while the memory is fresh and she is available.
Lead to Opportunity
Having a letter of recommendation on hand means you have what you need to apply for various opportunities, including scholarships, grants or recognitions. Most things you have to apply for or compete with your colleagues over, require a letter of recommendation. Having the letter may boost your confidence and support your professional development. Not to mention, just having the letter in hand should help you feel better about your boss and make you happier knowing your boss supports you.