Well the generic correct answer is for "future proofing". CAT5 has already been on the way out in favor of CAT6's improved throughput; the same goes for the newer specs. I would never recommend installing the oldest/lowest spec. or getting the highest/newest either.
So to your 2nd question: Why lock in at the lowest spec currently on its way out ? Be ahead of the curve to save possible time, money and headaches down the road; think ahead IMHO; but most people buy for the bottom line price of today's market, which is OK.
I myself would not want to re-wire or upgrade again and again. Same reason I always run dedicated 2" conduit on the first wall, attic or basement cable run and I will always have easy access to add lines; and I generally always run at least one extra cable per each type for easy expansion; CAT, phone, RG6, component, HDMI, etc. Depending on the application.
I would also make sure to buy shielded cables; and in-wall rated.
Unshielded and shielded twisted pair cabling standards
* Cat 1: Currently unrecognized by TIA/EIA. Previously used for POTS telephone communications, ISDN and doorbell wiring.
* Cat 2: Currently unrecognized by TIA/EIA. Previously was frequently used on 4 Mbit/s token ring networks.
* Cat 3: Currently defined in TIA/EIA-568-B, used for data networks using frequencies up to 16 MHz. Historically popular for 10 Mbit/s Ethernet networks.
* Cat 4: Currently unrecognized by TIA/EIA. Provided performance of up to 20 MHz, and was frequently used on 16 Mbit/s token ring networks.
* Cat 5: Currently unrecognized by TIA/EIA. Provided performance of up to 100 MHz, and was frequently used on 100 Mbit/s ethernet networks. May be unsuitable for 1000BASE-T gigabit ethernet.
* Cat 5e: Currently defined in TIA/EIA-568-B. Provides performance of up to 100 MHz, and is frequently used for both 100 Mbit/s and gigabit ethernet networks.
* Cat 6: Currently defined in TIA/EIA-568-B. It provides performance of up to 250 MHz, more than double category 5 and 5e.
* Cat 6a: Future specification for 10 Gbit/s applications.
* Cat 7: An informal name applied to ISO/IEC 11801 Class F cabling. This standard specifies four individually-shielded pairs (STP) inside an overall shield. Designed for transmission at frequencies up to 600 MHz.