I do not believe it is necasary but it all depends on your beliefs. A lot of beginner books suggest making an altar to help with focus on your spell. Casting a spell is largely about focus of your will.
Your spells work when you focus on your beliefs and believe that the spell will work. An altar or other items are not necassary unless you feel the need for them. Some people cannot afford to make an altar. However, others use natural items or things they find.
By my understanding an alter is more-or-less just a formalized place where you keep and maintain any tools or other magic-working items. It is meant to be a space you can dedicate to the purpose of magic. It is where you would place idols of the god/s you follow or recognize (or sacrifice to), a place to keep/burn your incense, lay out crystals, keep your books and journals, your book of shadows, etc.
In a nutshell it is a magical version of being your 'office'.