[1] Rama 581/3 in name of Minhagim; Hashlama of Reb Nechemia 581/2

[2] The reasons:

1. To make a separation between the Tekios of the custom and those Biblically required on Rosh Hashanah. [Levush 581/1; Taz 581/4; M”A 581/14; Hashlama of Reb Nechemia 581/2]
2. To confuse the Satan so he think that R”H has already passed. [M”A ibid in name of Minhagim; Elya Raba 581; Hashlama of Reb Nechemia 581/2]
3. To be able to say Shehechiyanu on the Shofar on Rosh Hashanah. [Chochmas Shlomo 581; Ashel Avraham 581]
4. To show that the next day is Rosh Hashanah. [Chochmas Shlomo 581]

The practical ramification-Blowing at home: The M”A ibid writes that according to the first reason one may blow the Shofar for practice in the privacy of his home. However according to the second reason of the Minhagim one must completely avoid blowing the Shofar at all. 

 [3] M”A 581/14

[4] M”B 581/24; Kaf Hachaim 581/77

The reason: As by reciting the words “Zichron Teruah” on Shabbos it is considered as if the Shofar was blown. This especially applies due to the fact that in the Mikdash the Shofar was blown. [ibid]

[5] M”A ibid; P”M 581 A”A 14

[6] Elya Raba 581/4 in name of Amrakel; First opinion in M”A 581/14; P”M 581 A”A 14; M”B 581/24; Ben Ish Chaiy Netzavim 2 [“One who needs to practice is to do so in a closed room or in a pit”]; Hashlama of Reb Nechemia 581/2

 Other opinions: The M”A ibid writes that according to the reason that this custom is done in order to confuse the Satan, then one may not blow at all. This implies that one may not blow even for practice and even in a closed room. However from the P”M ibid and M”B ibid it implies that even according to this reason in a closed room it is permitted to blow for practice.

[7] Kaf Hachaim 581/76

[8] So is implied from Poskim ibid to suspect for the second reason of the Minhagim as rules the M”A ibid.